## Why
Narrow completion popups can hide distinguishing filenames, parent paths, or category tags. Unified mentions also need a stable result viewport across searches and filters.
## What changed
- Use shared selection styling and `›` markers across command, file, skill, and unified mention popups.
- Give unified mentions a fixed-height picker layout with a header, filter tabs, query display, scroll indicators, and keyboard hints.
- Bound mention name columns to leave room for parent paths and measure alignment in terminal cells.
- Truncate file paths while retaining filenames and recalculating visible fuzzy highlights, preserving the full path for insertion.
- Keep category tags visible when skill and app descriptions are hidden.
## Testing
Add snapshots and assertions for narrow and clipped layouts, scrolling, empty filters, full-row selection styling, Unicode column alignment, long paths, preserved file selections, and category tags.
GitOrigin-RevId: 54c34cc2472b43bac22011206efed3041eb77308
e28fa3e7a4