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/mol_stale.go

@@ -136,7 +136,7 @@ func findStaleMolecules(ctx context.Context, s storage.Storage, blockingOnly, un
 	}
 
 	// Get blocked issues to find what each stale molecule is blocking
-	blockedIssues, err := s.GetBlockedIssues(ctx)
+	blockedIssues, err := s.GetBlockedIssues(ctx, types.WorkFilter{})
 	if err != nil {
 		return nil, fmt.Errorf("querying blocked issues: %w", err)
 	}