[Feat] Promote candidates into parent worktree

[samzong]

What's changed?

• Add gmc wt dup --task for copying task context files into duplicate candidates.
• Change gmc wt promote to apply the selected candidate back into the current parent worktree, with --dry-run support.
• Reject dirty parent worktrees and skip unchanged copied task files during promotion.

Why

• Supports the parallel candidate workflow without hidden dirty-parent bypass behavior or branch-rename promotion semantics.