Lifts `visible_slash_menu_entries`, `apply_slash_menu_selection`, and
`try_autocomplete_slash_command` from `tui/ui.rs` into a sibling
module. Drops the now-unused `slash_completion_hints` import from
`ui.rs` (the new module imports it directly).
Kept separate from `tui::file_mention` per the audit doc — the two
popups have distinct trigger characters, ranking, and post-selection
behaviour even though they share UI scaffolding.
`ui.rs`: ~5070 → ~4990 lines.
Workspace tests: 1011/1011 still green.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
25cfe11736