cmd/git(feat[GitWorktreeManager]): add GitWorktreeManager and GitWorktreeCmd classes

why: Enable management of git worktrees using the Manager/Cmd pattern
what:
- Add GitWorktreeCmd class with remove, lock, unlock, move, repair methods
- Add GitWorktreeManager class with add, prune, ls, get, filter methods
- Parse porcelain output to extract worktree_path, head, branch, locked, prunable
- Add worktrees property to Git class for easy access

Author: tony