feat(beads): add attachment fields for pinned beads (gt-rana.1)

Implement AttachmentFields to track molecule attachments on pinned/handoff beads:
- AttachedMolecule: root issue ID of attached molecule
- AttachedAt: timestamp when attached

API:
- AttachMolecule(pinnedBeadID, moleculeID) - attach
- DetachMolecule(pinnedBeadID) - detach
- GetAttachment(pinnedBeadID) - query
- ParseAttachmentFields(issue) - parse from description
- FormatAttachmentFields(fields) - format for description

Includes comprehensive tests for parsing and API.

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

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

internal/cmd/molecule.go

@@ -131,6 +131,44 @@ Example:
 	RunE: runMoleculeProgress,
 }
 
+var moleculeAttachCmd = &cobra.Command{
+	Use:   "attach <pinned-bead-id> <molecule-id>",
+	Short: "Attach a molecule to a pinned bead",
+	Long: `Attach a molecule to a pinned/handoff bead.
+
+This records which molecule an agent is currently working on. The attachment
+is stored in the pinned bead's description and visible via 'bd show'.
+
+Example:
+  gt molecule attach gt-abc mol-xyz`,
+	Args: cobra.ExactArgs(2),
+	RunE: runMoleculeAttach,
+}
+
+var moleculeDetachCmd = &cobra.Command{
+	Use:   "detach <pinned-bead-id>",
+	Short: "Detach molecule from a pinned bead",
+	Long: `Remove molecule attachment from a pinned/handoff bead.
+
+This clears the attached_molecule and attached_at fields from the bead.
+
+Example:
+  gt molecule detach gt-abc`,
+	Args: cobra.ExactArgs(1),
+	RunE: runMoleculeDetach,
+}
+
+var moleculeAttachmentCmd = &cobra.Command{
+	Use:   "attachment <pinned-bead-id>",
+	Short: "Show attachment status of a pinned bead",
+	Long: `Show which molecule is attached to a pinned bead.
+
+Example:
+  gt molecule attachment gt-abc`,
+	Args: cobra.ExactArgs(1),
+	RunE: runMoleculeAttachment,
+}
+
 func init() {
 	// List flags
 	moleculeListCmd.Flags().BoolVar(&moleculeJSON, "json", false, "Output as JSON")
@@ -154,6 +192,9 @@ func init() {
 	// Progress flags
 	moleculeProgressCmd.Flags().BoolVar(&moleculeJSON, "json", false, "Output as JSON")
 
+	// Attachment flags
+	moleculeAttachmentCmd.Flags().BoolVar(&moleculeJSON, "json", false, "Output as JSON")
+
 	// Add subcommands
 	moleculeCmd.AddCommand(moleculeListCmd)
 	moleculeCmd.AddCommand(moleculeShowCmd)
@@ -162,6 +203,9 @@ func init() {
 	moleculeCmd.AddCommand(moleculeInstancesCmd)
 	moleculeCmd.AddCommand(moleculeExportCmd)
 	moleculeCmd.AddCommand(moleculeProgressCmd)
+	moleculeCmd.AddCommand(moleculeAttachCmd)
+	moleculeCmd.AddCommand(moleculeDetachCmd)
+	moleculeCmd.AddCommand(moleculeAttachmentCmd)
 
 	rootCmd.AddCommand(moleculeCmd)
 }
@@ -818,3 +862,120 @@ func extractMoleculeID(description string) string {
 	}
 	return ""
 }
+
+func runMoleculeAttach(cmd *cobra.Command, args []string) error {
+	pinnedBeadID := args[0]
+	moleculeID := args[1]
+
+	workDir, err := findLocalBeadsDir()
+	if err != nil {
+		return fmt.Errorf("not in a beads workspace: %w", err)
+	}
+
+	b := beads.New(workDir)
+
+	// Attach the molecule
+	issue, err := b.AttachMolecule(pinnedBeadID, moleculeID)
+	if err != nil {
+		return fmt.Errorf("attaching molecule: %w", err)
+	}
+
+	attachment := beads.ParseAttachmentFields(issue)
+	fmt.Printf("%s Attached %s to %s\n", style.Bold.Render("✓"), moleculeID, pinnedBeadID)
+	if attachment != nil && attachment.AttachedAt != "" {
+		fmt.Printf("  attached_at: %s\n", attachment.AttachedAt)
+	}
+
+	return nil
+}
+
+func runMoleculeDetach(cmd *cobra.Command, args []string) error {
+	pinnedBeadID := args[0]
+
+	workDir, err := findLocalBeadsDir()
+	if err != nil {
+		return fmt.Errorf("not in a beads workspace: %w", err)
+	}
+
+	b := beads.New(workDir)
+
+	// Check current attachment first
+	attachment, err := b.GetAttachment(pinnedBeadID)
+	if err != nil {
+		return fmt.Errorf("checking attachment: %w", err)
+	}
+
+	if attachment == nil {
+		fmt.Printf("%s No molecule attached to %s\n", style.Dim.Render("ℹ"), pinnedBeadID)
+		return nil
+	}
+
+	previousMolecule := attachment.AttachedMolecule
+
+	// Detach the molecule
+	_, err = b.DetachMolecule(pinnedBeadID)
+	if err != nil {
+		return fmt.Errorf("detaching molecule: %w", err)
+	}
+
+	fmt.Printf("%s Detached %s from %s\n", style.Bold.Render("✓"), previousMolecule, pinnedBeadID)
+
+	return nil
+}
+
+func runMoleculeAttachment(cmd *cobra.Command, args []string) error {
+	pinnedBeadID := args[0]
+
+	workDir, err := findLocalBeadsDir()
+	if err != nil {
+		return fmt.Errorf("not in a beads workspace: %w", err)
+	}
+
+	b := beads.New(workDir)
+
+	// Get the issue
+	issue, err := b.Show(pinnedBeadID)
+	if err != nil {
+		return fmt.Errorf("getting issue: %w", err)
+	}
+
+	attachment := beads.ParseAttachmentFields(issue)
+
+	if moleculeJSON {
+		type attachmentOutput struct {
+			IssueID          string `json:"issue_id"`
+			IssueTitle       string `json:"issue_title"`
+			Status           string `json:"status"`
+			AttachedMolecule string `json:"attached_molecule,omitempty"`
+			AttachedAt       string `json:"attached_at,omitempty"`
+		}
+		out := attachmentOutput{
+			IssueID:    issue.ID,
+			IssueTitle: issue.Title,
+			Status:     issue.Status,
+		}
+		if attachment != nil {
+			out.AttachedMolecule = attachment.AttachedMolecule
+			out.AttachedAt = attachment.AttachedAt
+		}
+		enc := json.NewEncoder(os.Stdout)
+		enc.SetIndent("", "  ")
+		return enc.Encode(out)
+	}
+
+	// Human-readable output
+	fmt.Printf("\n%s: %s\n", style.Bold.Render(issue.ID), issue.Title)
+	fmt.Printf("Status: %s\n", issue.Status)
+
+	if attachment == nil || attachment.AttachedMolecule == "" {
+		fmt.Printf("\n%s\n", style.Dim.Render("No molecule attached"))
+	} else {
+		fmt.Printf("\n%s\n", style.Bold.Render("Attached Molecule:"))
+		fmt.Printf("  ID: %s\n", attachment.AttachedMolecule)
+		if attachment.AttachedAt != "" {
+			fmt.Printf("  Attached at: %s\n", attachment.AttachedAt)
+		}
+	}
+
+	return nil
+}