Fix #723: Prevent poisoned role stamp during task capability denial

This fixes #723 Defect B where attempting to acquire a reviewer lease from a merger session caused the session role to be incorrectly stamped as 'reviewer', leading to downstream workspace binding exceptions.

This commit moves the preflight capability recording to only apply when the task is allowed, and adds a try/except downstream to correctly fail-closed instead of throwing RuntimeError.

gitea_mcp_server.py

@@ -4096,9 +4096,6 @@ def _evaluate_pr_review_submission(
     worktree_path: str | None = None,
 ) -> dict:
     """Shared gate chain for live submit and dry-run review tools."""
-    _verify_role_mutation_workspace(
-        remote, worktree_path=worktree_path, task="review_pr"
-    )
     action = (action or "").strip().lower()
     workflow_blockers = _review_workflow_load_gate_reasons() if live else []
     result = {
@@ -4115,6 +4112,13 @@ def _evaluate_pr_review_submission(
         "remote": remote if remote in REMOTES else None,
         "reasons": [],
     }
+    try:
+        _verify_role_mutation_workspace(
+            remote, worktree_path=worktree_path, task="review_pr"
+        )
+    except RuntimeError as e:
+        result["reasons"].append(str(e))
+        return result
     reasons = result["reasons"]
     if workflow_blockers:
         reasons.extend(workflow_blockers)
@@ -10018,9 +10022,20 @@ def gitea_adopt_merger_pr_lease(
             "permission_report": _permission_block_report("gitea.pr.merge"),
         }
 
-    _verify_role_mutation_workspace(
-        remote, worktree=worktree, task="adopt_merger_pr_lease"
-    )
+    try:
+        _verify_role_mutation_workspace(
+            remote, worktree=worktree, task="adopt_merger_pr_lease"
+        )
+    except RuntimeError as e:
+        return {
+            "success": False,
+            "adopted": False,
+            "pr_number": pr_number,
+            "reasons": [str(e)],
+            "active_lease": None,
+            "expected_head_sha": expected_head_sha,
+            "live_head_sha": None,
+        }
     h, o, r = _resolve(remote, host, org, repo)
     auth = _auth(h)
     profile = get_profile()
@@ -13921,8 +13936,6 @@ def gitea_resolve_task_capability(
                         "exact_safe_next_action": next_safe_action,
                     }
 
-    record_preflight_check("capability", required_role, resolved_task=task)
-
     # Try automatic dispatch switching
     _ensure_matching_profile(required_permission, required_role, remote, host)
 
@@ -13956,6 +13969,9 @@ def gitea_resolve_task_capability(
         permission_allowed_in_current_session and role_matches_current_session
     )
 
+    if allowed_in_current_session:
+        record_preflight_check("capability", required_role, resolved_task=task)
+
     switching = gitea_config.is_runtime_switching_enabled()
     available_in_session = allowed_in_current_session
     configured = False