feat: add conflict-fix leases and stale-head protection (Closes #399)

Introduce structured PR work leases so author conflict-fix pushes cannot
race reviewer validation, approval, or merge on a moving head SHA.

- pr_work_lease.py: parse/acquire leases, push and reviewer mutation gates
- gitea_acquire_conflict_fix_lease, gitea_assess_conflict_fix_push MCP tools
- Enforce reviewed head SHA on mark_final_review_decision, submit_pr_review, merge_pr
- Final-report rules and workflow sections 20A / 26B
- tests/test_pr_work_lease.py (14 cases)

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

@@ -732,6 +732,24 @@ 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)
+
+Before validating, approving, or merging a PR:
+
+1. Check for an active conflict-fix lease on the PR; stop if one is active.
+2. Pin `expected_head_sha` before validation and pass it to
+   `gitea_mark_final_review_decision`, `gitea_submit_pr_review`, and
+   `gitea_merge_pr`.
+3. Re-fetch live PR head immediately before approval and merge; refuse when
+   live head differs from the reviewed SHA.
+
+Final reports must state:
+
+* reviewed head SHA
+* final live head SHA before approval
+* final live head SHA before merge
+* whether any push occurred during validation
+
 ## 27. Merge rules
 
 Before merge, rerun fresh live checks:

skills/llm-project-workflow/workflows/work-issue.md

@@ -577,6 +577,29 @@ After push, report:
 
 If push fails, stop and produce a recovery handoff.
 
+## 20A. Conflict-fix lease and push gate (#399)
+
+When pushing to an existing PR branch to resolve merge conflicts:
+
+1. Call `gitea_acquire_conflict_fix_lease` before any push.
+2. Call `gitea_assess_conflict_fix_push` immediately before `git push` with:
+   * branch head before push
+   * branch head after push (local)
+   * session worktree path
+   * push cwd
+   * whether the push is fast-forward
+3. Do not push when a reviewer holds an active lease on the same PR.
+4. Do not force-push.
+5. Do not push from the main checkout or wrong cwd.
+
+Conflict-fix final reports must state:
+
+* branch head before push
+* branch head after push
+* active reviewer lease status
+* whether push was fast-forward
+* whether any reviewer was active
+
 ## 21. PR creation rules
 
 Create a PR only if implementation and validation pass, unless project policy explicitly allows draft PRs with documented validation failures.