fix(shell): index tracked vendor files

[lihailong00]

Summary

• allow Git-tracked files under vendor/ to participate in @ file completion
• continue filtering tracked node_modules and other generated directories
• continue excluding untracked vendor trees to avoid candidate-set growth

Reproduction and verification

Before the fix, a tracked vendor/dep.py was deterministically absent from completion. Regression tests also create 100 untracked vendor files and verify that they remain excluded.

• targeted file-completion/filter tests — 28 passed across focused runs
• targeted Ruff and Pyright — passed

Closes #1962

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.