feat: wt <worktree> is short for wt cd <worktree>

[lorenzolfm]

wt cd <worktree> is verbose for the command you type most often, so an unknown first word is now a worktree name. It reaches the same lookup as wt cd, so a branch name and the start of a directory name work in the short form as well.

Collisions. A command wins over a worktree with the same name, because clap reads the first word as a command before anything else. wt ls therefore prints the worktrees also when a worktree has the name ls, and the long form wt cd ls reaches that worktree. Three things keep the rule visible:

• wt add prints a warning when it makes a directory with the name of a command. The list of commands comes from clap itself, so it cannot drift.
• A word that is neither gives one message for both: there is no command or worktree with the name snyc.
• wt --help names the short form, and wt help cd gives the rule.

Shell. The fish function now names the commands that write for the user, and reads a path from every other word. The binary therefore decides what is a worktree. A worktree name also completes in the position of a command.

Tested in a throwaway repository: the short form, a branch name, a prefix, a worktree with the name ls, a mistyped command, and the fish function on each of them.