⚡ Bolt: Optimize path containment check in is_in_dir

[thirdeyenation]

💡 What: Replaced os.path.commonpath with a faster string-based prefix check str.startswith in is_in_dir in helpers/files.py.
🎯 Why: os.path.commonpath is a significant bottleneck due to internal list allocations and path splitting. Path containment checks are frequently used, so this small improvement has a wide impact.
📊 Impact: Expected performance improvement is ~3.6x speedup for the is_in_dir function.
🔬 Measurement: Verified using a local benchmark script testing 100k iterations of matching and non-matching paths.

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PR created automatically by Jules for task 10750840739437611190 started by @thirdeyenation

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