* fix(ui): prevent useEffect from running every render in InputPrompt
getDirectories() returns a new array reference each call, causing the
useEffect dependency check to fail on every render. Move the call
inside the effect body and use stable dependencies [config, dirs] so
the effect only re-runs when they actually change.
* fix(ui): use serialized dep key for directory change detection
Move from [config, dirs] deps (both stable refs that miss external
changes) to a dirKey string (join of current directories). This
preserves the perf fix (no new array ref in deps) while still
detecting directory additions/removals from /add-dir etc.
* refactor(cli): remove unused dirs state from InputPrompt
The dirs parameter passed to useCommandCompletion() was never read
inside that hook, making the dirs state and sync effect in InputPrompt
dead code. Remove the parameter, the state, the effect, and all test
call-site args.
