Add support for memory through Letta

[codeluggage]

I've been using Letta with agent-shell for a while and it's useful for the memory features.

Checklist

• I agree to communicate with the author myself (not AI-generated).
• I've read the README's Filing issues section.
• I'm running the latest versions (fill in below).
  • agent-shell version: 0.52.2
  • acp.el version: 0.12.2
  • ACP package (e.g. claude-agent-acp) version: 0.42.0 (claude-agent-acp), but running letta-code-acp 0.0.2
  • Agent CLI (e.g. claude, gemini) version: 0.26.7 (Letta Code)
• For requesting new agent support, I'm including a link to the ACP-capable agent or related ACP package.
• For issues, I'm including ACP traffic (as per README).