ci: security review: pass prompt as system prompt

diff --git a/contrib/ci/claude_security_review.py b/contrib/ci/claude_security_review.py
old mode 100755
new mode 100644
index 7324bff..342bf7a
--- a/contrib/ci/claude_security_review.py
+++ b/contrib/ci/claude_security_review.py
@@ -80,20 +80,22 @@ def changed_files_from_diff(diff: str) -> str:
     )
 
 
-def build_prompt(diff: str, changed_files: str, commit_messages: str) -> str:
+def read_system_prompt() -> str:
     with open(PROMPT_FILE) as f:
-        instructions = f.read()
+        return f.read()
 
+
+def build_user_prompt(diff: str, changed_files: str, commit_messages: str) -> str:
     return (
-        f"{instructions}\n\n"
-        f"---\n\n"
+        "Review the following PR diff according to the review "
+        "guidelines in your system prompt.\n\n"
         f"## Changed files\n\n```\n{changed_files}\n```\n\n"
         f"## Commit messages\n\n```\n{commit_messages}\n```\n\n"
         f"## Diff\n\n```diff\n{diff}\n```"
     )
 
 
-def run_claude(prompt: str) -> str | None:
+def run_claude(user_prompt: str, system_prompt: str) -> str | None:
     """Invoke Claude Code CLI in print mode. Returns review text or None on failure.
 
     Passes the prompt via stdin to avoid OS argument length limits (MAX_ARG_STRLEN).
@@ -105,12 +107,13 @@ def run_claude(prompt: str) -> str | None:
         "--model", CLAUDE_MODEL,
         "--effort", CLAUDE_EFFORT,
         "--output-format", "text",
+        "--append-system-prompt", system_prompt,
     ]
 
     try:
         result = subprocess.run(
             cmd,
-            input=prompt,
+            input=user_prompt,
             capture_output=True,
             text=True,
             timeout=CLAUDE_TIMEOUT_SECONDS,
@@ -237,10 +240,11 @@ def main() -> int:
         print(f"ERROR: diff is {len(diff)} chars, exceeds maximum of {MAX_DIFF_CHARS}. Skipping review.")
         return 2
 
-    prompt = build_prompt(diff, changed_files, commit_messages)
+    user_prompt = build_user_prompt(diff, changed_files, commit_messages)
+    system_prompt = read_system_prompt()
 
     print(f"\nRunning Claude Code review (model: {CLAUDE_MODEL})...\n")
-    review = run_claude(prompt)
+    review = run_claude(user_prompt, system_prompt)
 
     if review is None:
         print("Review failed to produce output.")
diff --git a/contrib/ci/security_review_prompt.md b/contrib/ci/security_review_prompt.md
index 5a417d4..e6fbce3 100644
--- a/contrib/ci/security_review_prompt.md
+++ b/contrib/ci/security_review_prompt.md
@@ -8,10 +8,10 @@ developer time and erodes trust in this review.
 
 ## Scope
 
-Focus your findings on the diff provided below -- only flag issues introduced or worsened by
-changes in this PR. You have access to the full Electrum codebase; use it freely to read
-surrounding code, trace call chains, and understand what the diff actually does. But do not
-audit code outside the diff -- the codebase is context, not the review target.
+Focus your findings on the diff provided in the user message -- only flag issues introduced
+or worsened by changes in this PR. You have access to the full Electrum codebase; use it
+freely to read surrounding code, trace call chains, and understand what the diff actually
+does. But do not audit code outside the diff -- the codebase is context, not the review target.
 Focus on changes that introduce, worsen, or fail to mitigate security vulnerabilities.
 Only flag issues introduced or worsened by the diff. Do not flag
 pre-existing issues visible in context lines unless the change makes them newly exploitable.