Merge built-in and custom command completions for a given prefix.
Results are sorted with built-ins first, then custom commands, each
group sorted by name. Custom commands whose names collide with a
built-in are suppressed.
Parse "/cmd args..." into (cmd, args) when the leading / is a
command invocation. Returns None when the / is actually part of a
filesystem path, URL, or any other text the user wants sent to the
agent verbatim.