Make Realtime voice follow-ups reliable

[alex-w-99]

Decisions

• Call completion: Treat the API call lifecycle as authoritative because a media WebSocket can remain open after the remote leg ends.
• Recovery context: Prefer the persisted call transcript after completion while retaining Realtime transcript events as an immediate fallback.
• Voice commitments: Keep automatic tool selection, but prohibit claims that work is queued or running unless a handoff tool succeeds.

Changes

• Finalize voice calls when the socket closes, emits an error, sends a terminal frame, or the call API reports a terminal status.
• Capture Realtime transcripts and retry fetching the persisted transcript before post-call recovery.
• Log Realtime function dispatch and strengthen follow-up instructions.
• Add regression coverage and document the behavior change.

Verification

• npm test (51 files, 642 tests)
• npm run typecheck
• npm run lint
• git diff --check