fix: resolve conflicts for PR #416

Merge current prgs/master into feat/issue-399-conflict-fix-leases.
Preserve reviewer PR lease gate (#407) before conflict-fix stale-head
gate (#399); document both as §26B and §26C. Align MCP and validator
tests with mandatory expected_head_sha and stale-head proof fields.

skills/llm-project-workflow/workflows/review-merge-pr.md

@@ -732,7 +732,27 @@ The final report must identify:
 * whether same-PR merge continuation was allowed
 * whether the run stopped as required
 
-## 26B. Conflict-fix lease and stale-head protection (#399)
+## 26B. Per-PR reviewer lease (#407)
+
+Parallel reviewer sessions are allowed only when each session holds a distinct,
+live PR lease.
+
+Before validation or review mutation on a selected PR:
+
+1. Call `gitea_acquire_reviewer_pr_lease` with worktree path, candidate head SHA,
+   and target branch SHA.
+2. Post heartbeats via `gitea_heartbeat_reviewer_pr_lease` before validation,
+   after validation, before review mutation, and before merge.
+3. Do not approve, request changes, or merge unless the in-session lease
+   matches the selected PR.
+
+If PR head or target branch advances during the lease, stop and refresh
+inventory before continuing.
+
+Final reports must include lease session id, acquisition proof, heartbeat
+status, and release/blocked status.
+
+## 26C. Conflict-fix lease and stale-head protection (#399)
 
 Before validating, approving, or merging a PR: