fix(mcp): make capability resolver side-effect free (Closes #685)

Stale-runtime detection remains fail-closed, but gitea_resolve_task_capability
must never touch mcp_config.json, spawn recovery threads, or call os._exit.

- Remove _trigger_mcp_auto_restart from the read-only diagnostics path
- Return blocker_kind=runtime_reconnect_required with mutation_performed=false
- Keep exact_safe_next_action pointing at IDE/client reconnect when stale
- Add regression suite across author/reviewer/merger/reconciler profiles

tests/test_mcp_stale_runtime.py

@@ -111,21 +111,13 @@ class TestMcpStaleRuntime(unittest.TestCase):
         reasons = gitea_mcp_server._check_mcp_runtimes_diagnostics("create_issue", ["prgs-author"])
         self.assertTrue(any("stale-runtime: The active Gitea MCP server process is stale" in r for r in reasons))
 
-    @patch("threading.Thread")
-    @patch("os.utime")
-    @patch("os.path.exists")
-    @patch.dict("os.environ", {"MCP_CONFIG_PATH": "/tmp/mcp_config.json"})
-    def test_auto_restart_trigger_touches_and_spawns(self, mock_exists, mock_utime, mock_thread):
-        mock_exists.return_value = True
-        gitea_mcp_server._restart_triggered = False
-
-        # Ensure we are not skipped in test mode for testing purposes
-        with patch("gitea_mcp_server._preflight_in_test_mode", return_value=False):
-            gitea_mcp_server._trigger_mcp_auto_restart()
-
-        mock_utime.assert_called_once_with("/tmp/mcp_config.json", None)
-        mock_thread.assert_called_once()
-        self.assertTrue(gitea_mcp_server._restart_triggered)
+    def test_auto_restart_helper_removed_from_read_only_path(self):
+        """#685: config-touch / os._exit self-recovery is no longer on the server."""
+        self.assertFalse(
+            hasattr(gitea_mcp_server, "_trigger_mcp_auto_restart"),
+            "_trigger_mcp_auto_restart must not remain (side-effect-free resolver)",
+        )
+        self.assertFalse(hasattr(gitea_mcp_server, "_restart_triggered"))
 
 
 if __name__ == "__main__":