When a Telegram group is upgraded to a supergroup, the chat ID changes
(e.g., -123456 → -100123456). This causes the bot to lose its group
configuration since it's keyed by chat ID.
This change:
- Adds handler for `message:migrate_to_chat_id` event
- Logs the migration (old_id → new_id) for visibility
- If the old chat ID has config in channels.telegram.groups, automatically:
- Copies the config to the new chat ID
- Removes the old chat ID entry
- Saves the updated config file
This eliminates the need to manually update clawdbot.json when groups
migrate to supergroups.
