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/types/types.go

@@ -677,6 +677,19 @@ type TreeNode struct {
 	Truncated bool   `json:"truncated"`
 }
 
+// MoleculeProgressStats provides efficient progress info for large molecules.
+// This uses indexed queries instead of loading all steps into memory.
+type MoleculeProgressStats struct {
+	MoleculeID    string     `json:"molecule_id"`
+	MoleculeTitle string     `json:"molecule_title"`
+	Total         int        `json:"total"`           // Total steps (direct children)
+	Completed     int        `json:"completed"`       // Closed steps
+	InProgress    int        `json:"in_progress"`     // Steps currently in progress
+	CurrentStepID string     `json:"current_step_id"` // First in_progress step ID (if any)
+	FirstClosed   *time.Time `json:"first_closed,omitempty"`
+	LastClosed    *time.Time `json:"last_closed,omitempty"`
+}
+
 // Statistics provides aggregate metrics
 type Statistics struct {
 	TotalIssues              int     `json:"total_issues"`