chore(auto-merge): sync to canonical template

Neutral wording in the header comment; no functional change. The
same-repo guard on enable-auto-merge is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ANcpLua

.github/workflows/auto-merge.yml

@@ -25,11 +25,10 @@
 #   - claude/  — Claude Code agents
 #
 # Both jobs require head.repo.full_name == github.repository. A branch prefix
-# is chosen by whoever opens the PR, so under pull_request_target — which runs
-# with the base repo's write token even for fork PRs — the prefix alone gates
-# nothing. Without the same-repo check any fork can push a `claude/…` branch
-# and land it on main unreviewed (ancplua-claude-plugins#345, merged 10s after
-# open by an outside contributor while the policy trusted `codex/`).
+# is chosen by whoever opens the PR, and pull_request_target runs with the base
+# repo's write token even for fork PRs — so the prefix alone gates nothing.
+# Without the same-repo check, any fork can push a `claude/…` branch and land
+# it on main unreviewed.
 
 name: Auto-merge