feat: wt add goes to the worktree that has the branch

[lorenzolfm]

Summary

• `wt add ` failed with `fatal: '' is already used by worktree at ...` when a worktree had the branch already.
• The command now looks for the branch in the worktree list first. When it finds one, it prints that worktree's path, so the shell cds there. This works when the directory name differs from the branch name.
• README describes the behavior under "Make a worktree".

Test plan

• `cargo build`, `cargo clippy`, `cargo test` (34 pass)
• In a scratch bare repo: `wt add feat/x mydir`, then `wt add feat/x` prints `mydir (branch feat/x is here already)` and the path, exit 0