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]>

skills/llm-project-workflow/schemas/pr-queue-cleanup-final-report.md

@@ -0,0 +1,31 @@
+# PR-queue-cleanup final report schema
+
+One report per cleanup run (one run = one PR). Fields must all be present;
+use `none` where nothing occurred. Validated by
+`pr_queue_cleanup.assess_pr_queue_cleanup_report` (fail closed).
+
+* Task: pr-queue-cleanup
+* Workflow source: workflows/pr-queue-cleanup.md (+ version/commit/hash)
+* Repo:
+* Role/profile:
+* Identity:
+* PR inventory pagination proof: (inventory_complete / final page / total_count)
+* Queue ordering proof:
+* Earlier PRs skipped: (with live per-PR proof)
+* Selected PR: (exactly one)
+* Pinned head SHA:
+* Review decision: (single terminal decision)
+* Merge authorized for PR: true/false (explicit per-PR operator authorization)
+* Merge gates result:
+* Merge result: (none / not attempted / merged SHA / blocker)
+* Run stop point: (which §4 chain rule ended the run)
+* Next suggested PR: (named, not continued to)
+* File edits by reviewer:
+* Worktree/index mutations:
+* Git ref mutations:
+* MCP/Gitea mutations:
+* Issue mutations: none (required — forbidden in cleanup mode)
+* Branch mutations: none (required — forbidden in cleanup mode)
+* Read-only diagnostics:
+* Blockers:
+* Safe next action: (fresh run for the next PR)