⚡ Bolt: Optimize path containment checks

[thirdeyenation]

💡 What: Replaced os.path.commonpath with os.path.abspath and str.startswith() with conditionally appended os.sep in is_in_dir inside helpers/files.py.
🎯 Why: os.path.commonpath is a significant bottleneck due to internal list allocations and path component string splitting.
📊 Impact: Path containment checks are ~3-4x faster, which benefits overall file system operations speed.
🔬 Measurement: Verified with timing scripts; startswith executes in ~0.24s compared to ~0.73s for 100,000 iterations.

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PR created automatically by Jules for task 5160876674549811961 started by @thirdeyenation

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