[Bug] File picker excludes untracked and gitignored files when git is available

Bug Description
The @ file picker no longer shows files from the filesystem — it appears to rely on git ls-files instead. This means untracked files, .gitignored files (like .env), and any file in a repo with no commits are completely invisible in the picker. Notably, if there is no .git directory at all, the fallback to filesystem listing works correctly and all files are shown. This is a regression: users frequently need to reference files that aren't yet committed — a new file being drafted, an environment config excluded from version control, or simply working in a fresh repo before the first commit. The file picker should fall back to filesystem listing when git is unavailable or incomplete, and ideally show all files in the working directory regardless of their git status, since the picker is a UI convenience for referencing context, not a git tool.

Environment Info

• Platform: darwin
• Terminal: iTerm.app
• Version: 2.1.45
• Feedback ID: 6af6bf2b-2c5b-4e08-9c86-95254ede19bf