Require manual reviewer assignment

diff --git a/.forgejo/workflows/assign-reviewer.yml b/.forgejo/workflows/assign-reviewer.yml
index 2e1ffb7..b8fd938 100644
--- a/.forgejo/workflows/assign-reviewer.yml
+++ b/.forgejo/workflows/assign-reviewer.yml
@@ -1,21 +1,18 @@
 name: Assign a random reviewer
 
 # Forgejo has no built-in random/round-robin reviewer assignment (only
-# path-based CODEOWNERS), so pick a random developer for each newly opened
-# pull request and request their review via the API.
+# path-based CODEOWNERS), so this workflow picks a random developer on manual
+# request.
 #
-# Runs automatically when a PR is opened, and can also be run manually
-# (workflow_dispatch) against any PR number to pick an additional reviewer.
+# Runs through workflow_dispatch against any PR number to pick the first or an
+# additional reviewer. Forgejo's contributor-facing "Assign random reviewer"
+# button dispatches this workflow for contributors without workflow access.
 #  * Someone already requested as a reviewer, or who has already submitted a
 #    review, is never picked.
-#  * On automatic runs (including re-runs), nothing is done at all if anyone
-#    from the REVIEWERS pool has already reviewed or been requested (reviews
-#    from people outside the pool don't count). Manual runs skip this check
-#    and always add a reviewer if an eligible candidate remains.
+#  * Contributors outside the reviewer pool can request only the first
+#    reviewer. Pool members can request additional reviewers.
 
 on:
-  pull_request_target:
-    types: [opened]
   workflow_dispatch:
     inputs:
       pr:
@@ -39,13 +36,11 @@ jobs:
           echo "jwt=$jwt" >> "$FORGEJO_OUTPUT"
       - name: Request review from a random developer
         # This never checks out or runs any PR code -- it only makes API
-        # calls -- so running in the base-repo context (pull_request_target,
-        # which is what grants the token write access even for fork PRs) is safe.
+        # calls, so running in the base-repo context is safe.
         env:
           API: ${{ github.server_url }}/api/v1
           REPO: ${{ github.repository }}
-          EVENT_NAME: ${{ github.event_name }}
-          EVENT_PR: ${{ github.event.pull_request.number }}
+          ACTOR: ${{ github.actor }}
           INPUT_PR: ${{ github.event.inputs.pr }}
           # Space-separated pool of candidate reviewers.
           REVIEWERS: "matt val wpaulino joostjager jkczyz benthecarman tankyleo tnull"
@@ -53,11 +48,7 @@ jobs:
           set -eu
           AUTH="Authorization: bearer ${{ steps.jwt.outputs.jwt }}"
 
-          if [ "$EVENT_NAME" = "workflow_dispatch" ]; then
-            PR="$INPUT_PR"
-          else
-            PR="$EVENT_PR"
-          fi
+          PR="$INPUT_PR"
           case "$PR" in
             ''|*[!0-9]*) echo "Invalid PR number: '$PR'"; exit 1 ;;
           esac
@@ -87,12 +78,11 @@ jobs:
           INVOLVED="$(printf '%s\n%s\n' "$REQUESTED" "$REVIEWED" |
             awk -v author="$AUTHOR" '$0 != "" && $0 != author' | sort -u)"
 
-          # On automatic runs, if a pool member is already on the PR there is
-          # nothing to do. Manual runs go ahead and add another reviewer.
-          if [ "$EVENT_NAME" != "workflow_dispatch" ]; then
+          # Only pool members may request an additional reviewer.
+          if ! printf '%s\n' $REVIEWERS | grep -qxF "$ACTOR"; then
             for d in $REVIEWERS; do
               if printf '%s\n' "$INVOLVED" | grep -qxF "$d"; then
-                echo "$d has already reviewed or been requested on PR #$PR; nothing to do."
+                echo "$d is already on PR #$PR; only pool members may request another reviewer."
                 exit 0
               fi
             done
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ad25fb1..cf7dd6a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -116,6 +116,14 @@ test out the patch set and opine on the technical merits of the patch. PR should
 be reviewed first on the conceptual level before focusing on code style or
 grammar fixes.
 
+Before requesting human review, wait for AI to complete its initial review
+and address its feedback. Once the pull request is ready, use the "Assign random
+reviewer" button in Forgejo.
+
+Most pull requests require one human reviewer. The primary reviewer may request
+a second reviewer for complex changes, such as changes to the channel state
+machine, or when there is design uncertainty.
+
 Coding Conventions
 ------------------