Keep individual skills out of the top-level slash command menu so large
skill collections do not crowd out built-in commands.
Skills still complete after `/skill`, including both the full skill list
after `/skill ` and prefix matches after `/skill <prefix>`.
(cherry picked from commit 57f8e3ad84dad9cf46290c0dc23e2b26504196df)
reidliu41
46ab1fdf62