Implements cost tracking v1 for Gas Town agents:
- `gt costs` shows live costs scraped from running tmux sessions
- `gt costs --today/--week` shows historical costs from ledger
- `gt costs --by-role/--by-rig` shows cost breakdowns
- `gt costs record` for Stop hook integration to record final costs
Cost ledger stored at ~/.gt/costs.jsonl with JSONL entries per session.
Ref: gt-nrpiq
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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