docs: create new document describing proposed release/branch management

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+# Release Branch Management
+
+## Overview
+
+This document describes the branch management workflow for lnd releases. The
+master branch remains open for merges at all times. Release stabilization
+happens on dedicated release branches, with CI automation handling backports of
+milestone-tagged changes. This approach maintains continuous development
+velocity while ensuring stable releases.
+
+## Branch Model Principles
+
+The release process operates on four core principles:
+
+**Master is always open.** Developers merge approved pull requests at any time
+without coordination around release windows. No merge freezes occur.
+
+**Each major release gets a dedicated branch.** When cutting a new major
+version, create a release branch from master. This branch handles all release
+candidates and subsequent patch releases for that version series.
+
+**CI automation handles backports.** Pull requests merged to master and tagged
+with a release milestone are automatically backported to the corresponding
+release branch. The automation creates backport PRs when conflicts occur.
+
+**Changes flow one direction only.** Changes move from master to release
+branches, never in reverse. Master always represents the latest development
+state.
+
+## Master Branch
+
+The master branch contains ongoing development work for future releases. It
+never freezes for releases.
+
+### Master Version Convention
+
+Master uses a `.99` patch version to indicate unreleased development work. After
+creating the `v0.21.x-branch` branch, update master's version in
+`build/version.go` to `0.21.99-beta`. This clearly signals post-0.21 but
+pre-0.22 code.
+
+When creating the next release branch (`v0.22.x-branch`), update master to
+`0.22.99-beta`. This pattern sorts correctly and is immediately recognizable as
+a development build.
+
+### Merging to Master
+
+Developers merge to master following normal review processes. If a change should
+be included in an active or upcoming release, tag the pull request with the
+appropriate milestone (`v0.21.0`, `v0.21.1`, etc.). The CI automation handles
+backporting after merge.
+
+No special coordination is required. Merge whenever the PR is approved,
+regardless of ongoing release activities. Initially manual input may be required
+to resolve conflicts that may arise. In the future LLM bots can help alleviate
+this manual work.
+
+## Major Release Process
+
+A major release introduces new features and represents a new minor version
+(e.g., 0.21.0, 0.22.0).
+
+### Creating the Release Branch
+
+When ready to begin a major release:
+
+1. Create a release branch from master: `git checkout -b v0.21.x-branch master`
+2. Push the branch: `git push origin v0.21.x-branch`
+3. Update `build/version.go` on the release branch to `0.21.0-beta.rc1`
+4. Commit the version bump: `git commit -am "build: bump version to v0.21.0-beta.rc1"`
+5. Update master's version to `0.21.99-beta` via a pull request
+6. Configure branch protection for `v0.21.x-branch` on GitHub
+
+### Release Candidate Cycle
+
+Create the first release candidate by tagging the version bump commit:
+
+```bash
+git tag -s v0.21.0-beta.rc1 -m "lnd v0.21.0-beta.rc1"
+git push origin v0.21.0-beta.rc1
+```
+
+This triggers CI to build release artifacts and Docker images.
+
+As testing uncovers issues, develop fixes on master and tag them with the
+`v0.21.0` milestone. Once merged, CI automation backports them to
+`v0.21.x-branch`. If backports apply cleanly, they merge automatically. If
+conflicts occur, CI creates backport PRs for manual resolution.
+
+When ready for the next release candidate:
+
+1. Create a pull request against the release branch to update `build/version.go` to `0.21.0-beta.rc2`
+2. After merging the PR, tag the merge commit on the release branch: `git tag -s v0.21.0-beta.rc2 -m "lnd v0.21.0-beta.rc2"`
+3. Push the new tag: `git push origin v0.21.0-beta.rc2`
+
+Repeat this cycle (rc3, rc4, etc.) until the release is stable.
+
+### Final Release
+
+For the final release, remove the RC suffix:
+
+1. Create a pull request against the release branch to update `build/version.go` to `0.21.0-beta`
+2. After merging the PR, tag the merge commit on the release branch: `git tag -s v0.21.0-beta -m "lnd v0.21.0-beta"`
+3. Push the new tag: `git push origin v0.21.0-beta`
+
+The `v0.21.x-branch` branch now enters maintenance mode for future patch
+releases.
+
+## Minor Release Process
+
+Minor (patch) releases fix bugs or security issues in released versions. They
+reuse the existing release branch for that version series.
+
+### Creating a Patch Release
+
+When a critical fix is needed for version 0.21.0:
+
+1. Develop and merge the fix to master
+2. Tag the PR with the `v0.21.1` milestone
+3. CI automation backports to `v0.21.x-branch`
+4. Create a pull request against the release branch to update `build/version.go` to `0.21.1-beta.rc1`
+5. After merging the PR, tag the merge commit: `git tag -s v0.21.1-beta.rc1 -m "lnd v0.21.1-beta.rc1"`
+6. Push the new tag: `git push origin v0.21.1-beta.rc1`
+
+If additional fixes are needed, follow the same process, incrementing through
+rc2, rc3, etc.
+
+For the final patch release:
+
+1. Create a pull request against the release branch to update `build/version.go` to `0.21.1-beta`
+2. After merging the PR, tag the merge commit: `git tag -s v0.21.1-beta -m "lnd v0.21.1-beta"`
+3. Push the new tag: `git push origin v0.21.1-beta`
+
+Multiple patch releases (0.21.1, 0.21.2, 0.21.3) can be created on the same
+`v0.21.x-branch` branch throughout the version's lifetime.
+
+## Manual Cherry-Picking
+
+Occasionally, a fix may be needed on a release branch that doesn't apply to
+master (release-specific issues, backports to older versions where master has
+diverged significantly, etc.).
+
+### When to Cherry-Pick Manually
+
+Cherry-pick directly to a release branch when:
+
+- The issue only exists on the release branch, not on master
+- Master's code has changed significantly, making a direct backport impractical
+- An urgent hotfix is needed before CI automation completes
+
+### Cherry-Pick Process
+
+```bash
+# Switch to the release branch
+git checkout v0.21.x-branch
+
+# Cherry-pick the commit from master
+git cherry-pick <commit-hash>
+
+# If conflicts occur, resolve them and continue
+git cherry-pick --continue
+```
+
+Cherry-picks still follow the normal PR flow, so a PR should be made only into
+the target release branch for normal review and CI.
+
+When manually cherry-picking, document why the normal backport flow was
+bypassed. If a corresponding change is needed on master (to prevent the bug from
+reappearing in future releases), ensure it's merged there as well.
+
+## Pull Request Milestones
+
+Developers use GitHub milestones to indicate which releases should include their
+changes.
+
+### Assigning Milestones
+
+When opening a PR, consider whether it should be backported to an active
+release:
+
+- **Bug fixes for active releases:** Assign the major release milestone (e.g.,
+`v0.21.0`)
+
+- **Critical fixes for older versions:** Assign the patch release milestone
+(e.g., `v0.20.3`)
+
+- **Features for future releases only:** No milestone, or assign the next major
+release milestone
+
+Milestones can be assigned at any time, even after merge. CI automation
+processes milestone-tagged PRs whenever they're detected.
+
+### Multiple Milestones
+
+If a fix needs to go into multiple release branches, assign multiple milestones
+to the PR. CI handles each backport independently. For example, a security fix
+might get both `v0.21.0` and `v0.20.3` milestones.
+
+## Backport Automation
+
+CI automation monitors merged PRs and backports milestone-tagged changes to the
+appropriate release branches. This section describes the automation's behavior.
+Implementation details are tracked in separate GitHub issues.
+
+### Automatic Backports
+
+When a PR with a release milestone merges to master:
+
+1. CI detects the milestone and identifies the target release branch
+2. CI attempts a three-way merge onto the release branch
+3. If successful, CI commits directly with a reference to the original PR
+4. The backported change appears in the next release candidate
+
+Developers don't need to take any action for successful backports.
+
+### Conflict Resolution
+
+When a backport conflicts:
+
+1. CI creates a new PR against the release branch
+2. The PR contains the attempted backport with conflict markers
+3. CI assigns the PR to the original author and notifies via GitHub mentions
+4. The author or maintainers resolve conflicts and merge the backport PR
+
+Backport PRs follow the normal review process and must pass all CI checks.
+
+### Monitoring Backports
+
+Track backport status through GitHub Projects or by filtering PRs. Backport PRs
+include labels indicating the original PR and milestone. Successfully backported
+commits reference the original PR in their commit messages.
+
+## Version Bump Timing
+
+Version numbers in `build/version.go` must be updated at specific points in the
+release process.
+
+**On release branches:** Update immediately before tagging. The commit that
+updates the version is the commit that gets tagged. This ensures built binaries
+report the correct version.
+
+**On master:** Update when creating a new release branch. Master moves from
+`0.20.99-beta` to `0.21.99-beta` when `v0.21-release` is created.
+
+**For each RC:** Increment the RC number before tagging. `0.21.0-beta.rc1` →
+`0.21.0-beta.rc2` → `0.21.0-beta.rc3`, etc.
+
+**For final releases:** Remove the RC suffix. `0.21.0-beta.rc5` → `0.21.0-beta`.
+
+## Branch Model Visualization
+
+The following diagrams illustrate the branch workflow and change flow.
+
+### Timeline View: Major Release Branch Lifecycle
+
+```mermaid
+gitGraph
+    commit id: "0.20.0 released"
+    commit id: "Feature A"
+    commit id: "Feature B"
+    branch v0.21.x-branch
+    commit id: "Version → 0.21.0-rc1" tag: "v0.21.0-beta.rc1"
+    checkout main
+    commit id: "Feature C (for 0.22)"
+    commit id: "Feature D (for 0.22)"
+    checkout v0.21.x-branch
+    commit id: "Backport: Fix X"
+    commit id: "Version → 0.21.0-rc2" tag: "v0.21.0-beta.rc2"
+    checkout main
+    commit id: "Feature E (for 0.22)"
+    commit id: "Fix Y (backport to 0.21)"
+    checkout v0.21.x-branch
+    commit id: "Backport: Fix Y"
+    commit id: "Version → 0.21.0" tag: "v0.21.0-beta"
+    checkout main
+    commit id: "Feature F (for 0.22)"
+    commit id: "Feature G (for 0.22)"
+    checkout v0.21.x-branch
+    commit id: "Backport: Critical fix Z"
+    commit id: "Version → 0.21.1-rc1" tag: "v0.21.1-beta.rc1"
+    commit id: "Version → 0.21.1" tag: "v0.21.1-beta"
+    checkout main
+    commit id: "Continue development"
+```
+
+After the v0.21.x-branch branch is created, both branches evolve independently.
+Master continues with features for future releases while the release branch
+focuses solely on stabilization and bug fixes.
+
+### Pull Request Flow with Milestone-Based Backports
+
+```mermaid
+flowchart TD
+    A[PR Merged to Master] --> B{Has Release<br/>Milestone?}
+    B -->|No| C[Done - Stays in Master Only]
+    B -->|Yes| D{Release Branch<br/>Exists?}
+    D -->|No| E[Queued for Future<br/>Release Branch]
+    D -->|Yes| F{First RC<br/>Tagged?}
+    F -->|No| G[Queued Until<br/>RC1 Tagged]
+    F -->|Yes| H[CI: Attempt<br/>Backport]
+    H --> I{Clean<br/>Apply?}
+    I -->|Yes| J[Auto-merge to<br/>Release Branch]
+    I -->|No| K[Create Backport PR<br/>for Manual Resolution]
+    J --> L[Done]
+    K --> M[Maintainer Resolves<br/>Conflicts]
+    M --> L
+```
+
+The milestone tag triggers the backport process. CI validates that the target
+release branch exists and has entered the RC phase before attempting backports.
+
+### Major vs Minor Release Branching
+
+```mermaid
+gitGraph
+    commit id: "Development"
+    commit id: "More work"
+    branch v0.21.x-branch
+    commit id: "v0.21.0-rc1" tag: "v0.21.0-beta.rc1"
+    commit id: "v0.21.0-rc2" tag: "v0.21.0-beta.rc2"
+    commit id: "v0.21.0 final" tag: "v0.21.0-beta"
+    checkout main
+    commit id: "Continue dev"
+    commit id: "More features"
+    checkout v0.21.x-branch
+    commit id: "Patch fix 1"
+    commit id: "v0.21.1" tag: "v0.21.1-beta"
+    commit id: "Patch fix 2"
+    commit id: "v0.21.2" tag: "v0.21.2-beta"
+    checkout main
+    commit id: "Keep developing"
+    branch v0.22.x-branch
+    commit id: "v0.22.0-rc1" tag: "v0.22.0-beta.rc1"
+    checkout main
+    commit id: "Future work"
+```
+
+The v0.21.x-branch branch serves both the initial 0.21.0 release and subsequent
+patch releases (0.21.1, 0.21.2). When 0.22 development is ready, a new
+v0.22.x-branch branch is created, and the cycle repeats.