feat: add mv/rename command for worktree renaming

[kawarimidoll]

Problem

When a worktree's branch name no longer reflects its purpose (e.g., feature-wip should become feature-auth), users must manually:

1. Rename the branch with git branch -m
2. Move the worktree directory with git worktree move
3. Ensure both operations succeed or manually rollback

Proposed Solution

Add a git gtr mv (or git gtr rename) command that renames both the worktree directory and its associated branch in a single operation.

git gtr mv feature-wip feature-auth
git gtr mv old-name new-name --force  # for locked worktree

I'm happy to implement this if needed :)