Add `bd compact --dolt` command to run Dolt garbage collection on
.beads/dolt directory. This helps reclaim disk space when using the
Dolt backend, where auto-commit per mutation causes commit history
to grow over time.
Features:
- Runs `dolt gc` in the .beads/dolt directory
- Shows disk space before/after with bytes freed
- Supports --dry-run to preview without running GC
- Supports --json for machine-readable output
- Helpful error messages for missing dolt directory or command
Closes: hq-ew1mbr.14
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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