Feature request: support @-style file lookup in the composer

## Feature request

Support `@`-style file lookup in the chat composer so users can quickly reference files by name/path while composing a prompt.

## Why this would help

When working on large codebases, it is much faster to type something like:

- `@src/core/SessionManager.ts`
- `@README.md`
- `@app/(shell)/modal/audio-prompt-input.tsx`

than to switch to a separate attach-file flow or manually describe the path from memory.

This is especially useful for ACP-connected coding agents where the prompt often needs precise file context.

## Requested behavior

When typing `@` in the composer:
- show a file/path picker/autocomplete
- search project files by path/name
- insert the selected file reference into the prompt
- ideally support keyboard navigation

## Notes

This request is specifically about fast file lookup/mention UX in the chat composer.
It is separate from attached-file chips or edited-file summaries.

Start by locating the chat composer entry point and reviewing how prompt text is edited today. Use the example paths in the issue as acceptance cases: typing @ should find files by path or name, support selection and keyboard navigation, and insert the chosen reference into the prompt.

Written by the indexing model from the issue text.