⚡ Optimize file content extraction performance

[myaple]

💡 What: Optimized extract_relevant_file_sections in src/repo_context.rs by changing the intermediate lines storage from Vec<String> to Vec<&str>, avoiding string allocation for every line in the file. Also hoisted the to_lowercase() call for keywords out of the inner loop to prevent repeated allocations.

🎯 Why: The previous implementation allocated a new String for every line in the file content, which is memory intensive for large files. It also re-allocated lowercased keywords for every line iteration.

📊 Measured Improvement:

• Benchmark added in src/tests/repo_context_perf_test.rs.
• Baseline: ~30.4ms for 100,000 lines.
• Optimized: ~10.9ms for 100,000 lines.
• ~3x performance improvement in extraction time.

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PR created automatically by Jules for task 4761616264722925510 started by @myaple

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