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)

final_report_validator.py

@@ -490,6 +490,45 @@ def _rule_reviewer_validation_failure_history(
     ]
 
 
+def _rule_reviewer_stale_head_proof(report_text: str) -> list[dict[str, str]]:
+    from pr_work_lease import assess_reviewer_stale_head_final_report
+
+    result = assess_reviewer_stale_head_final_report(report_text)
+    if result.get("proven"):
+        return []
+    return _findings_from_reasons(
+        "reviewer.stale_head_proof",
+        result.get("reasons") or [],
+        field="Stale-head proof",
+        severity="block",
+        safe_next_action=(
+            "state reviewed head SHA, live head before approval/merge, and "
+            "whether any push occurred during validation"
+        ),
+    )
+
+
+def _rule_conflict_fix_push_proof(report_text: str) -> list[dict[str, str]]:
+    from pr_work_lease import assess_conflict_fix_final_report
+
+    text = report_text or ""
+    if "conflict-fix" not in text.lower() and "conflict fix" not in text.lower():
+        return []
+    result = assess_conflict_fix_final_report(text)
+    if result.get("proven"):
+        return []
+    return _findings_from_reasons(
+        "author.conflict_fix_push_proof",
+        result.get("reasons") or [],
+        field="Conflict-fix push proof",
+        severity="block",
+        safe_next_action=(
+            "state branch head before/after push, reviewer lease status, "
+            "fast-forward status, and whether any reviewer was active"
+        ),
+    )
+
+
 def _rule_reviewer_validation_command(report_text: str) -> list[dict[str, str]]:
     text = report_text or ""
     if not _BARE_PYTEST_RE.search(text):
@@ -909,6 +948,7 @@ _RULES_BY_TASK: dict[str, list[Callable[..., list[dict[str, str]]]]] = {
         _rule_reviewer_target_branch_freshness,
         _rule_reviewer_mutation_ledger,
         _rule_reviewer_review_mutation,
+        _rule_reviewer_stale_head_proof,
     ],
     "reconcile_already_landed": [
         _rule_reconcile_controller_handoff,
@@ -930,6 +970,7 @@ _RULES_BY_TASK: dict[str, list[Callable[..., list[dict[str, str]]]]] = {
         _rule_shared_controller_handoff,
         _rule_shared_email_disclosure,
         _rule_reviewer_vague_mutations_none,
+        _rule_conflict_fix_push_proof,
     ],
     "issue_filing": [
         _rule_shared_controller_handoff,