refactor: remove unused Beads.Pin/Unpin methods (bd-x0zl)

These methods called `bd pin` and `bd unpin` which have been removed.
Neither method was ever called - gt uses `bd update --status=pinned` instead.

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

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

internal/beads/beads.go

@@ -394,29 +394,6 @@ func (b *Beads) CloseWithReason(reason string, ids ...string) error {
 	return err
 }
 
-// Pin pins an issue to an agent's hook.
-// This sets the pinned boolean field and optionally the assignee.
-// If agent is empty, only sets the pinned field.
-// NOTE: There's a known issue (gt-o3is) where the pinned field gets overwritten
-// by subsequent bd commands due to a bug in beads auto-import logic.
-// The handoff bead attachment mechanism works correctly and is the primary
-// work assignment mechanism. The pinned field is cosmetic for bd hook visibility.
-func (b *Beads) Pin(id string, agent string) error {
-	args := []string{"--no-daemon", "pin", id}
-	if agent != "" {
-		args = append(args, "--for="+agent)
-	}
-	_, err := b.run(args...)
-	return err
-}
-
-// Unpin removes the pinned flag from an issue.
-func (b *Beads) Unpin(id string) error {
-	args := []string{"unpin", id, "--json"}
-	_, err := b.run(args...)
-	return err
-}
-
 // Release moves an in_progress issue back to open status.
 // This is used to recover stuck steps when a worker dies mid-task.
 // It clears the assignee so the step can be claimed by another worker.