feat: recover stale sessions on gateway startup

Scan all session files on startup and recover any with stale turn state
(pending_user_turn or runtime_checkpoint metadata) left from a previous
crash or restart. Previously this only happened lazily when a new message
arrived for the affected session.

- Add SessionManager.list_session_keys() to enumerate session keys from disk
- Add AgentLoop.recover_stale_sessions() to scan and recover all sessions
- Call recover_stale_sessions() during gateway startup in _run_gateway

nanobot/agent/loop.py

@@ -2038,3 +2038,34 @@ class AgentLoop:
         finally:
             await self._runtime_events().run_status_changed(msg, session_key, "idle")
             self._runtime_events().clear_turn(session_key)
+
+    def recover_stale_sessions(self) -> int:
+        """Scan all sessions on startup and recover any with stale turn state.
+
+        Returns the number of sessions that were recovered.
+        """
+        recovered = 0
+        for key in self.sessions.list_session_keys():
+            try:
+                session = self.sessions.get_or_create(key)
+                changed = False
+                if self._restore_runtime_checkpoint(session):
+                    changed = True
+                if self._restore_pending_user_turn(session):
+                    changed = True
+                if changed:
+                    self.sessions.save(session)
+                    recovered += 1
+                    logger.info(
+                        "Recovered stale session {} on startup",
+                        key,
+                    )
+            except Exception:
+                logger.debug(
+                    "Could not recover stale session {}",
+                    key,
+                    exc_info=True,
+                )
+        if recovered:
+            logger.info("Recovered {} stale session(s) on startup", recovered)
+        return recovered

nanobot/cli/commands.py

@@ -1744,6 +1744,10 @@ def _run_gateway(
         local_trigger_store=trigger_store,
         hook_factories=[create_file_edit_activity_hook],
     )
+
+    # Recover any sessions left in a stale state from a previous crash/restart.
+    agent.recover_stale_sessions()
+
     webui_turn_coordinator = WebuiTurnCoordinator(
         bus=bus,
         sessions=session_manager,

nanobot/session/manager.py

@@ -965,3 +965,23 @@ class SessionManager:
                     )
                 continue
         return sorted(sessions, key=lambda x: x.get("updated_at", ""), reverse=True)
+
+    def list_session_keys(self) -> list[str]:
+        """List all session keys from disk without loading full sessions."""
+        keys: list[str] = []
+        for path in self.sessions_dir.glob("*.jsonl"):
+            storage_key = self._session_key_from_path(path)
+            if storage_key is None:
+                continue
+            try:
+                with open(path, encoding="utf-8") as f:
+                    first_line = f.readline().strip()
+                    if first_line:
+                        data = json.loads(first_line)
+                        if isinstance(data, dict) and data.get("_type") == "metadata":
+                            keys.append(data.get("key") or storage_key)
+                        else:
+                            keys.append(storage_key)
+            except (OSError, _SESSION_DATA_ERRORS):
+                continue
+        return keys