Recent composer cleanups split state ownership out of ChatComposer, but slash-command handling still mixed parsing, popup coordination, completion, submission validation, queue behavior, and argument element rebasing into the main composer file. Pending changes to slash command parsing and selection inspired this code move to prevent chat_composer.rs bloat.

This is just a refactor, no functional or behavioral changes are intended.

What changed

• Move slash-command parsing and lookup helpers into bottom_pane/chat_composer/slash_input.rs.
• Move slash popup key handling, command-name completion, and popup construction into the slash input helper module.
• Centralize bare-command, inline-args, submission-validation, and queued-input action selection behind slash-specific helpers.
• Move command argument text-element rebasing into the slash input module so inline command submission keeps the same element behavior with less composer-local logic.

Verification

• just fmt
• just test -p codex-tui
• cargo insta pending-snapshots -p codex-tui