⚡ Bolt: Optimize path containment checks in is_in_dir

[thirdeyenation]

💡 What: Replaced os.path.commonpath with string comparison in is_in_dir.
🎯 Why: os.path.commonpath is slow because it relies on list allocations and path splitting. String comparison achieves the same outcome but faster.
📊 Impact: Reduces execution time for is_in_dir by ~70% (from ~0.72s to ~0.20s for 100k iterations).
🔬 Measurement: Run test_commonpath.py (added locally) and unit tests for helpers.files.

---

PR created automatically by Jules for task 16549282381726297701 started by @thirdeyenation

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.