Add lettabot-history CLI (#211)

* Add lettabot-history CLI

* Document and test lettabot-history

* Validate lettabot-history limit

* fix: address review feedback on history CLI

- Extract shared loadLastTarget into cli/shared.ts (was duplicated in message.ts, react.ts, history-core.ts)
- Clamp --limit to platform maximums (Discord: 100, Slack: 1000)
- Fix Discord author formatting: use globalName/username instead of deprecated discriminator
- Add Slack fetch test

Written by Cameron ◯ Letta Code

"You miss 100% of the shots you don't take." -- Wayne Gretzky -- Michael Scott

---------

Co-authored-by: Jason Carreira <jason@visotrust.com>
Co-authored-by: Cameron <cameron@pfiffer.org>

docs/README.md

@@ -8,6 +8,7 @@ LettaBot is a multi-channel AI assistant powered by [Letta](https://letta.com) t
 - [Self-Hosted Setup](./selfhosted-setup.md) - Run with your own Letta server
 - [Configuration Reference](./configuration.md) - All config options
 - [Commands Reference](./commands.md) - Bot commands reference
+- [CLI Tools](./cli-tools.md) - Agent/operator CLI tools
 - [Scheduling Tasks](./cron-setup.md) - Cron jobs and heartbeats
 - [Gmail Pub/Sub](./gmail-pubsub.md) - Email notifications integration
 - [Railway Deployment](./railway-deploy.md) - Deploy to Railway