feat(gateway): notify /restart requester when gateway comes back online

When a user sends /restart, the gateway now persists their routing info
(platform, chat_id, thread_id) to .restart_notify.json. After the new
gateway process starts and adapters connect, it reads the file, sends a
'Gateway restarted successfully' message to that specific chat, and
cleans up the file.

This follows the same pattern as _send_update_notification (used by
/update). Thread IDs are preserved so the notification lands in the
correct Telegram topic or Discord thread.

Previously, after /restart the user had no feedback that the gateway was
back — they had to send a message to find out. Now they get a proactive
notification and know their session continues.

gateway/run.py

@@ -1717,6 +1717,9 @@ class GatewayRunner:
         ):
             self._schedule_update_notification_watch()
 
+        # Notify the chat that initiated /restart that the gateway is back.
+        await self._send_restart_notification()
+
         # Drain any recovered process watchers (from crash recovery checkpoint)
         try:
             from tools.process_registry import process_registry
@@ -4147,6 +4150,22 @@ class GatewayRunner:
                 return f"⏳ Draining {count} active agent(s) before restart..."
             return "⏳ Gateway restart already in progress..."
 
+        # Save the requester's routing info so the new gateway process can
+        # notify them once it comes back online.
+        try:
+            import json as _json
+            notify_data = {
+                "platform": event.source.platform.value if event.source.platform else None,
+                "chat_id": event.source.chat_id,
+            }
+            if event.source.thread_id:
+                notify_data["thread_id"] = event.source.thread_id
+            (_hermes_home / ".restart_notify.json").write_text(
+                _json.dumps(notify_data)
+            )
+        except Exception as e:
+            logger.debug("Failed to write restart notify file: %s", e)
+
         active_agents = self._running_agent_count()
         self.request_restart(detached=True, via_service=False)
         if active_agents:
@@ -6880,6 +6899,48 @@ class GatewayRunner:
 
         return True
 
+    async def _send_restart_notification(self) -> None:
+        """Notify the chat that initiated /restart that the gateway is back."""
+        import json as _json
+
+        notify_path = _hermes_home / ".restart_notify.json"
+        if not notify_path.exists():
+            return
+
+        try:
+            data = _json.loads(notify_path.read_text())
+            platform_str = data.get("platform")
+            chat_id = data.get("chat_id")
+            thread_id = data.get("thread_id")
+
+            if not platform_str or not chat_id:
+                return
+
+            platform = Platform(platform_str)
+            adapter = self.adapters.get(platform)
+            if not adapter:
+                logger.debug(
+                    "Restart notification skipped: %s adapter not connected",
+                    platform_str,
+                )
+                return
+
+            metadata = {"thread_id": thread_id} if thread_id else None
+            await adapter.send(
+                chat_id,
+                "♻ Gateway restarted successfully. Your session continues.",
+                metadata=metadata,
+            )
+            logger.info(
+                "Sent restart notification to %s:%s",
+                platform_str,
+                chat_id,
+            )
+        except Exception as e:
+            logger.warning("Restart notification failed: %s", e)
+        finally:
+            notify_path.unlink(missing_ok=True)
+
     def _set_session_env(self, context: SessionContext) -> list:
         """Set session context variables for the current async task.