@ file picker should support fuzzy filename search in additionalDirectories

Description

When a directory is added via permissions.additionalDirectories in settings.json, the @ file picker can browse into the directory (e.g., typing @/Users/me/Downloads/ shows the folder), but it does not support fuzzy filename search within that directory the way it does for files inside the git repo.

Expected behavior

Typing @ and then part of a filename (e.g., report) should match files in additionalDirectories the same way it matches files in the project root — with fuzzy search and ranked results.

Current behavior

• @/Users/me/Downloads/ shows the directory exists
• Typing a filename after that path returns no results
• Files inside the git repo work fine with fuzzy search

Workaround

Users can paste the full path manually (@/Users/me/Downloads/exact-filename.csv) or ask Claude to find files via Bash.

Environment

• Claude Code CLI (macOS)
• additionalDirectories configured in ~/.claude/settings.json