feat: add custom prompts support

[Afur]

Summary

• Adds support for Codex CLI custom prompts inside CodexMonitor.
• Typing "/" now suggests available custom prompts (from ~/.codex/prompts) with descriptions and argument hints.
• Selecting a prompt inserts /prompts:<name> with argument placeholders and Tab navigation between args.
• Handles prompt expansion (named and positional) before sending, matching Codex CLI behavior.
• Improves file mention insertion inside prompt args and normalizes smart quotes for parsing.

Details

• Backend: new prompts_list Tauri command reads $CODEX_HOME/prompts (default ~/.codex/prompts), parses front matter, and returns prompt metadata.
• Frontend: new hook for prompt discovery, autocomplete trigger for /, and UI rendering for hints.
• Composer: Tab jumps between argument placeholders; @file inside an arg inserts without breaking parsing.
• Prompt expansion + validation are applied before turn start; invalid args block send with a user-facing error.

Demo

Screen.Recording.2026-01-14.mov

How to test

1. Create a prompt file in ~/.codex/prompts/example.md with front matter:

   ---
   description: Example prompt
   argument-hint: TITLE= DETAIL=
   ---
   Title: $TITLE
   Detail: $DETAIL
   

2. In the app, type / and confirm prompts:example appears with description + hint.
3. Select it; verify /prompts:example TITLE="" DETAIL="" inserts and Tab cycles through values.
4. Use @ inside an arg (e.g., DETAIL="See @README.md"), select a file, and confirm it inserts without replacing other text.
5. Send the prompt and verify expansion works; missing args should show a validation error and no turn starts.

Notes

• Custom prompts are loaded from Codex CLI config; app-server does not expose prompts, so listing happens client-side.
• README updated to mention /prompts:... autocomplete.