Add inbound attachment support with download, metadata, and pruning (#64)

* Add inbound attachment handling and pruning

* Add Signal attachment support and logging

- Implement full Signal attachment collection (copies from signal-cli dir)
- Add logging when attachments are saved to disk for all channels
- Skip audio attachments in Signal (handled by voice transcription)

Written by Cameron ◯ Letta Code

* Gitignore bun.lock

Keep lockfile local, don't track in repo.

Written by Cameron ◯ Letta Code

---------

Co-authored-by: Jason Carreira <jason@visotrust.com>

.gitignore

@@ -45,3 +45,4 @@ data/whatsapp-session/
 # Config with secrets
 lettabot.yaml
 lettabot.yml
+bun.lock