feat: add bd mol progress command for efficient molecule monitoring (bd-8xnf)

Adds a new `bd mol progress` command that shows molecule progress using
indexed queries instead of loading all steps into memory. This makes it
suitable for mega-molecules with millions of steps.

Features:
- Efficient SQL-based counting via idx_dependencies_depends_on_type index
- Progress display: completed / total (percentage)
- Current step identification
- Rate calculation from closure timestamps
- ETA estimation
- JSON output support

New storage interface method: GetMoleculeProgress(ctx, moleculeID)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/storage.go

@@ -126,6 +126,9 @@ type Storage interface {
 	// Statistics
 	GetStatistics(ctx context.Context) (*types.Statistics, error)
 
+	// Molecule progress (efficient for large molecules)
+	GetMoleculeProgress(ctx context.Context, moleculeID string) (*types.MoleculeProgressStats, error)
+
 	// Dirty tracking (for incremental JSONL export)
 	GetDirtyIssues(ctx context.Context) ([]string, error)
 	GetDirtyIssueHash(ctx context.Context, issueID string) (string, error) // For timestamp-only dedup (bd-164)