feat: add conversation management, persistence, and mobile app spec

Major enhancements to voice assistant with conversation persistence
and mobile app planning:

- Add conversation management with multiple active conversations
- Implement conversation persistence (save/load from disk)
- Add conversation selector UI for switching between conversations
- Enhance agent prompt with per-conversation tmux sessions
- Add mobile app technical specification (React Native + Expo)
- Improve UI with active processes view and agent stream
- Add ACP (Agent Control Protocol) server implementation

Mobile spec details:
- React Native with Expo SDK 54+
- NativeWind v5 for styling
- Share Zod schemas/types from server
- Android in-call experience (InCallManager)
- Direct WebSocket connection via Tailscale
- Same audio quality as web prototype

🤖 Generated with [Claude Code](https://claude.com/claude-code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

packages/voice-assistant/.gitignore

@@ -3,4 +3,5 @@ dist
 .env
 .DS_Store
 *.log
-.conversations
+.debug.conversations
+conversations