Voice mode: configurable silence timeout (re-filing #33792)

Re-filing per the auto-close notice on #33792 (closed for staleness Apr 22, locked May 1) — the bot explicitly said to file a new issue if relevant.

Use case where the 15s tap-mode silence cutoff hurts

Reviewing a video on a second screen while dictating critique back into Claude Code chat. Natural pauses between observations regularly exceed 15s — scrubbing back to verify a detail, watching a longer beat, or simply thinking before responding. The cutoff interrupts every natural reflection window in the workflow.

Workaround analysis

• Hold-Ctrl+O works when one hand is free. Breaks down when the same hand is operating the video player with a mouse — player controls and the hold-key compete for the same hand.
• Filling silence with "um um um" keeps the mic alive but contaminates the transcript and feels silly.

Both workarounds are ergonomically worse than the natural cadence the user wants.

Proposed fix

A configurable silence threshold via settings.json:

{
  "voice": {
    "silenceTimeoutMs": 5000
  }
}

Values from 1000 to 60000, plus "off" (or 0) to disable VAD entirely for users who'd rather end recording explicitly. This matches the VS Code pattern (accessibility.voice.speechTimeout) which users already know.

Reopening because #33792 was closed for staleness, not rejected on merits. The use case has gotten more relevant as people use Claude Code alongside other applications (video review, document review, dual-monitor workflows).