Add `pm cd <project> [<wt>]` for shell-side cwd shortcuts
Prints the project dir (or the forward symlink to a wt) so the caller
can wrap it: `cd "$(pm cd myproj wt1)"`. Errors clearly when the wt is
detached so a shell function doesn't silently land in the wrong place.
Wires zsh completion for both positionals via `_pm_wt_for_cd`, which
reads the project from `$words[3]` and falls back to `--project`.