feat(compact): add --dolt flag for Dolt garbage collection

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>

cmd/bd/admin_aliases.go

@@ -71,6 +71,7 @@ func init() {
 	compactAliasCmd.Flags().StringVar(&compactSummary, "summary", "", "Path to summary file (use '-' for stdin)")
 	compactAliasCmd.Flags().StringVar(&compactActor, "actor", "agent", "Actor name for audit trail")
 	compactAliasCmd.Flags().IntVar(&compactLimit, "limit", 0, "Limit number of candidates (0 = no limit)")
+	compactAliasCmd.Flags().BoolVar(&compactDolt, "dolt", false, "Dolt mode: run Dolt garbage collection on .beads/dolt")
 
 	// Reset alias flags - these read from cmd.Flags() in the Run function
 	resetAliasCmd.Flags().Bool("force", false, "Actually perform the reset (required)")