refactor: remove unused bd pin/unpin/hook commands (bd-x0zl)

Analysis found these commands are dead code:
- gt never calls `bd pin` - uses `bd update --status=pinned` instead
- Beads.Pin() wrapper exists but is never called
- bd hook functionality duplicated by gt mol status
- Code comment says "pinned field is cosmetic for bd hook visibility"

Removed:
- cmd/bd/pin.go
- cmd/bd/unpin.go
- cmd/bd/hook.go

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

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

cmd/bd/show_unit_helpers.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/steveyegge/beads/internal/storage"
+	"github.com/steveyegge/beads/internal/types"
+)
+
+func validateIssueUpdatable(id string, issue *types.Issue) error {
+	if issue == nil {
+		return nil
+	}
+	if issue.IsTemplate {
+		return fmt.Errorf("Error: cannot update template %s: templates are read-only; use 'bd molecule instantiate' to create a work item", id)
+	}
+	return nil
+}
+
+func validateIssueClosable(id string, issue *types.Issue, force bool) error {
+	if issue == nil {
+		return nil
+	}
+	if issue.IsTemplate {
+		return fmt.Errorf("Error: cannot close template %s: templates are read-only", id)
+	}
+	if !force && issue.Status == types.StatusPinned {
+		return fmt.Errorf("Error: cannot close pinned issue %s (use --force to override)", id)
+	}
+	return nil
+}
+
+func applyLabelUpdates(ctx context.Context, st storage.Storage, issueID, actor string, setLabels, addLabels, removeLabels []string) error {
+	// Set labels (replaces all existing labels)
+	if len(setLabels) > 0 {
+		currentLabels, err := st.GetLabels(ctx, issueID)
+		if err != nil {
+			return err
+		}
+		for _, label := range currentLabels {
+			if err := st.RemoveLabel(ctx, issueID, label, actor); err != nil {
+				return err
+			}
+		}
+		for _, label := range setLabels {
+			if err := st.AddLabel(ctx, issueID, label, actor); err != nil {
+				return err
+			}
+		}
+	}
+
+	// Add labels
+	for _, label := range addLabels {
+		if err := st.AddLabel(ctx, issueID, label, actor); err != nil {
+			return err
+		}
+	}
+
+	// Remove labels
+	for _, label := range removeLabels {
+		if err := st.RemoveLabel(ctx, issueID, label, actor); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}