feat(pr-sync): native assess and author update-by-merge lifecycle

Prevent approved PRs from stalling when master advances. Add
gitea_assess_pr_sync_status and gitea_update_pr_branch_by_merge with
head/base pinning, author-only updates, conflict handoff, and approval
invalidation after head changes. Wire task capability map, sequential
controller routing in review-merge workflow, and hermetic AC tests.

task_capability_map.py

@@ -64,6 +64,24 @@ TASK_CAPABILITY_MAP: dict[str, dict[str, str]] = {
         "permission": "gitea.branch.push",
         "role": "author",
     },
+    # PR synchronization lifecycle: assess is read-only (any role with gitea.read);
+    # update-by-merge is author-only and mutates the PR head via Gitea API.
+    "assess_pr_sync_status": {
+        "permission": "gitea.read",
+        "role": "author",
+    },
+    "gitea_assess_pr_sync_status": {
+        "permission": "gitea.read",
+        "role": "author",
+    },
+    "update_pr_branch_by_merge": {
+        "permission": "gitea.branch.push",
+        "role": "author",
+    },
+    "gitea_update_pr_branch_by_merge": {
+        "permission": "gitea.branch.push",
+        "role": "author",
+    },
     "review_pr": {
         "permission": "gitea.pr.review",
         "role": "reviewer",