perf: optimize file tree scanning for large repositories

[Karavil]

Summary

Use git ls-files instead of ripgrep for file tree generation (~7000x faster on large repos).

Problem

On large repos (42k+ files), file tree generation was taking 8+ seconds because ripgrep scanned ALL files before limiting output to 200.

Changes

• Add FileTree module using git ls-files --cached --others --exclude-standard
• Cache file tree at Instance level
• Remove Ripgrep.tree (replaced by FileTree.tree)

Performance (M3 MacBook Pro, 42k file repo)

• File tree: 8.39s → ~1ms

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