feat: add PR-only queue cleanup mode with per-PR dispatch gates (Closes #390)

Adds canonical pr-queue-cleanup workflow, report verifier, reviewer-only task
routing, and runbook guidance so operators can drain open PRs one canonical
review at a time with fail-closed boundaries on batching and unauthorized merges.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

docs/llm-workflow-runbooks.md

@@ -749,3 +749,22 @@ scripts/release-tag v0.4.0 --notes-file /tmp/release-notes.md --push
 - [`credential-isolation.md`](credential-isolation.md) — credential handling.
 - [`release-workflows.md`](release-workflows.md) — release/merge workflow.
 - [`../README.md`](../README.md) — canonical config, thin launchers, the menu.
+
+## PR-only queue cleanup mode (#390)
+
+Use `pr-queue-cleanup` mode when the queue holds many open PRs and the goal
+is to drain reviews deterministically. One run = one PR = one canonical
+review (`skills/llm-project-workflow/workflows/pr-queue-cleanup.md`).
+
+* Cleanup runs are reviewer-role only (`pr_queue_cleanup` resolver task);
+  author sessions get `wrong_role_stop`.
+* Forbidden in cleanup mode: issue claiming, branch creation, implementation
+  edits, new issue filing, and any second-PR review after a terminal
+  mutation.
+* Terminal chain: stop after `REQUEST_CHANGES`; after `APPROVED` continue
+  only to same-PR merge with explicit per-PR operator authorization and
+  passing gates; stop after merge or merge blocker.
+* Every run reports the next suggested PR without continuing to it; the next
+  PR requires a fresh run with fresh identity/capability/inventory proof.
+* Use full `review-merge-pr` mode instead when the operator wants a single
+  targeted review, and `work-issue` mode for any authoring.