refactor: improve session management, audio handling, and MCP configuration
- Change default session name from 'voice-dev' to '__voice-dev'
- Change default server port from 3000 to 6767
- Add conversation history persistence to .conversations directory
- Defer stream abort until after transcription completes
- Return empty content arrays from MCP tool handlers (use structuredContent only)
- Add pause/resume support to audio player for handling false positive speech detection
- Add .conversations to .gitignore
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