docs: add Tailscale access instructions for Letta
- Add comment to docker-compose.yml documenting Tailscale access
- Add "Letta Access via Tailscale" section to DEPLOY.md
- Include Letta ADE connection instructions and API examples
- Update architecture diagram to show Letta's Tailscale access
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>