Support absolute paths and `~` expansion in `@` file mentions/fuzzy search

What feature would you like to see?

Support absolute paths and ~ (home) expansion in @ file mentions / fuzzy file search in Codex TUI (and ideally aligned behavior in app-server fuzzy search).

Examples that should work:

• @/home/alice/project/src/main.rs
• @~/project/src/main.rs
• @~/project/src/mai (fuzzy completion)

Why this matters

Today, @ search works well for relative-in-workspace paths, but fails for common path forms users type from shell habit:

• absolute filesystem paths
• home-relative paths using ~

This creates friction when referencing files outside current relative context or when users naturally paste absolute paths from terminal/editor.

Current behavior (from source)

From codex-rs current main:

• @token query is forwarded as raw text (no path normalization / tilde expansion).
• file-search indexes and returns relative paths from search roots.
• selected popup match inserts that relative path.

So @~/... is treated literally and absolute-prefix query matching is not first-class.

Suggested behavior

1. Recognize and normalize path-like @ queries before fuzzy matching:

• expand ~ to user home
• detect absolute paths
• normalize separators and dot segments

1. Resolve query against active search roots:

• if query points inside a configured root, fuzzy-match the relative suffix within that root
• if query resolves outside current roots, either:
• show a clear "outside search roots" message, or
• optionally propose/add as an extra search root (future enhancement)

1. Preserve current behavior for plain fuzzy tokens (non path-like input).

Nice-to-have

• Add tests for:
• @~/... expansion
• absolute path inside root
• absolute path outside root
• mixed separator cases (Windows + POSIX)

• Align app-server fuzzy file search semantics with TUI for consistency.

Additional information

Related existing issues around @ search UX:

• #1261 (support @ fuzzy file search)
• #2867 (default list UX)
• #8863 (directory selection)
• #2952 (gitignore scope)