Refactor observe segment handoff to use draft folder pattern
Observers now own segment directories: create HHMMSS_draft/ folders during
recording, write files with simple names (audio.flac, center_DP-3_screen.webm),
then atomically rename to HHMMSS_LEN/ on completion. This eliminates complex
file movement in transcribe/describe handlers.
Key changes:
- Linux/macOS observers: draft folder creation and atomic rename on boundary
- transcribe/describe: process files in-place, skip if output exists
- sense: expect files in segment dirs, removed SEGMENT_KEY env var
- remote ingest: create segment dirs, strip legacy prefixes from filenames
- Removed unused is_draft_segment() and macOS filename properties
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>