feat: add long-form audio transcription

[ThomsenDrake]

Stack position

This is PR 3/5 in the upstream contribution stack.

Depends on:

• Stack PR #11

GitHub only allows stacked base branches inside the same repository, so this review slice targets the previous branch on the fork. After earlier slices land, this can be retargeted upstream.

Summary

• add audio_transcribe tool wiring for long-form transcription
• add chunking, overlap handling, merge/dedup behavior, and partial-failure handling
• add the desktop parity and config surface needed for the audio path

Out of scope

• workspace init and migration
• Chrome DevTools MCP
• unrelated provider or auth changes

Notes

• I have not rerun the full PR-specific test suite as part of PR creation