fix(mcp): complete merger lease acquire + structured capability resolve (#718, #723)

Harden gitea_acquire_merger_pr_lease with required candidate_head, live-head
match, and fail-closed POST handling. Add capability-map aliases for reviewer
and merger lease acquire/adopt. Make unknown tasks return structured
unknown_task (no ValueError escape). Expand tests for roles, head scoping,
aliases, and unknown_task.

Closes #718. Addresses #723 resolver unknown_task and lease task mapping.

task_capability_map.py

@@ -64,6 +64,14 @@ TASK_CAPABILITY_MAP: dict[str, dict[str, str]] = {
         "permission": "gitea.branch.push",
         "role": "author",
     },
+    "acquire_reviewer_pr_lease": {
+        "permission": "gitea.pr.comment",
+        "role": "reviewer",
+    },
+    "gitea_acquire_reviewer_pr_lease": {
+        "permission": "gitea.pr.comment",
+        "role": "reviewer",
+    },
     "review_pr": {
         "permission": "gitea.pr.review",
         "role": "reviewer",
@@ -86,10 +94,18 @@ TASK_CAPABILITY_MAP: dict[str, dict[str, str]] = {
         "permission": "gitea.pr.comment",
         "role": "merger",
     },
+    "gitea_adopt_merger_pr_lease": {
+        "permission": "gitea.pr.comment",
+        "role": "merger",
+    },
     "acquire_merger_pr_lease": {
         "permission": "gitea.pr.comment",
         "role": "merger",
     },
+    "gitea_acquire_merger_pr_lease": {
+        "permission": "gitea.pr.comment",
+        "role": "merger",
+    },
     # #691: guarded non-owner cleanup of obsolete comment-backed reviewer leases.
     # Apply path posts lease release + audit comments (gitea.pr.comment).
     "cleanup_obsolete_reviewer_comment_lease": {