Add segment key tracking to token usage logging
Track which recording segment (HHMMSS_LEN) triggered each AI call by:
- Reading SEGMENT_KEY env var in log_token_usage() and adding to token entries
- Setting SEGMENT_KEY in observe-transcribe, observe-describe, think-insight
- Passing SEGMENT_KEY via env in cortex_request for segment-scheduled agents
Also DRY the segment extraction logic:
- Add get_segment_key() and segment_and_suffix() helpers to observe/utils.py
- Refactor transcribe.py and describe.py to use shared helpers
- Remove duplicated _segment_and_suffix() from describe.py
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>