fix(restart): preserve channel metadata across /restart so reply lands in thread

cmd_restart only persisted channel + chat_id across the os.execv boundary, so
when the new process announced "Restart completed" the OutboundMessage had
no Slack thread_ts and the reply fell back to the channel root.

Serialize msg.metadata into NANOBOT_RESTART_NOTIFY_METADATA, restore it on the
RestartNotice, and forward it to OutboundMessage so the completion message
follows the same routing as the original /restart invocation.

Made-with: Cursor

nanobot/channels/manager.py

@@ -172,6 +172,7 @@ class ChannelManager:
                 channel=notice.channel,
                 chat_id=notice.chat_id,
                 content=format_restart_completed_message(notice.started_at_raw),
+                metadata=dict(notice.metadata or {}),
             ),
         ))