feat: Railway deployment support with one-click deploy (#106)

* feat: add Railway deployment support with agent auto-discovery

- Add railway.toml for build/deploy config with health checks
- Skip config file requirement when RAILWAY_ENVIRONMENT detected
- Auto-discover existing agent by name on container deploys
- Add findAgentByName() API function for agent lookup
- Add setAgentId() method to LettaBot class
- Add comprehensive Railway deployment docs

One-click deploy flow:
1. Set LETTA_API_KEY + channel tokens
2. LettaBot finds existing agent by AGENT_NAME (default: "LettaBot")
3. If not found, creates on first message
4. Subsequent deploys auto-reconnect to same agent

Written by Cameron ◯ Letta Code

"The best way to predict the future is to deploy it." - Railway, probably

* fix: specify Node 22 for Railway deployment

* fix: fail fast if LETTA_API_KEY is missing

* fix: don't await Telegram bot.start() - it never resolves

* fix: extract message from send_message tool call

* Revert "fix: extract message from send_message tool call"

This reverts commit 370306e49de3728434352d2df1b78c744e888833.

* fix: clear LETTA_AGENT_ID env var when agent doesn't exist

* docs: add Railway deploy button to README and docs

* fix: .nvmrc newline and correct MODEL default in docs

README.md

@@ -4,6 +4,8 @@ Your personal AI assistant that remembers everything across **Telegram, Slack, D
 
 <img width="750" alt="lettabot-preview" src="https://github.com/user-attachments/assets/9f01b845-d5b0-447b-927d-ae15f9ec7511" />
 
+[![Deploy on Railway](https://railway.app/button.svg)](https://railway.com/deploy/lettabot)
+
 ## Features
 
 - **Multi-Channel** - Chat seamlessly across Telegram, Slack, Discord, WhatsApp, and Signal