Add the voice helper lifecycle foundation

[copyberry]

Add the voice helper lifecycle foundation

What changed

• Add codex-realtime-webrtc with a length-prefixed, 256-byte JSON control protocol for hello, ready, close, and closed messages.
• Add a hardened codex-voice-host binary that validates protocol and build compatibility, handles orderly shutdown and parent pipe loss, and exposes its build commit.
• Keep this initial foundation limited to process lifecycle management; it does not open audio devices, load native plugins, negotiate WebRTC, or enable voice in the TUI.

Testing

• Exercise the helper as a subprocess, covering handshake, explicit close, parent pipe loss, malformed frames, incompatible builds, and invalid message order.

[github-actions]

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