fix: enforce merger role boundaries (Closes #539)

role_session_router.py

@@ -55,7 +55,6 @@ def skip_python_scan_walk_root(project_root: str, walk_root: str) -> bool:
 
 REVIEWER_TASKS = frozenset({
     "review_pr",
-    "merge_pr",
     "blind_pr_queue_review",
     "pr_queue_cleanup",
     "pr-queue-cleanup",
@@ -63,6 +62,10 @@ REVIEWER_TASKS = frozenset({
     "approve_pr",
 })
 
+MERGER_TASKS = frozenset({
+    "merge_pr",
+})
+
 AUTHOR_TASKS = frozenset({
     "create_issue",
     "comment_issue",
@@ -98,7 +101,7 @@ TASK_REQUIRED_ROLE = {
     "address_pr_change_requests": "author",
     "delete_branch": "author",
     "review_pr": "reviewer",
-    "merge_pr": "reviewer",
+    "merge_pr": "merger",
     "blind_pr_queue_review": "reviewer",
     "pr_queue_cleanup": "reviewer",
     "pr-queue-cleanup": "reviewer",
@@ -128,6 +131,10 @@ WRONG_ROLE_RECONCILER_MSG = (
     "MCP namespace/profile with exact close capability."
 )
 
+WRONG_ROLE_MERGER_MSG = (
+    "Wrong role/session for merger task. Launch merger MCP namespace."
+)
+
 _session_last_route: dict | None = None
 
 
@@ -243,6 +250,25 @@ def route_task_session(
         _record_route(result)
         return result
 
+    if required_role == "merger":
+        result = {
+            "task_type": task_type,
+            "required_role": required_role,
+            "active_role": active_role_kind,
+            "active_profile": active_profile,
+            "route_result": ROUTE_WRONG_ROLE,
+            "downstream_allowed": False,
+            "reasons": [
+                WRONG_ROLE_MERGER_MSG,
+                "Merger tasks cannot run in author or reviewer sessions.",
+            ],
+            "message": WRONG_ROLE_MERGER_MSG,
+            "runtime_switching_supported": runtime_switching_supported,
+            "profile_switch_blocked": not runtime_switching_supported,
+        }
+        _record_route(result)
+        return result
+
     if required_role == "author":
         route = ROUTE_TO_AUTHOR
         message = (