⚡ Bolt: Optimize file content aggregation in get_files.py

[georgi]

💡 What: Refactored get_files and get_content in src/nodetool/io/get_files.py to use list-based accumulation instead of string/list concatenation.
🎯 Why: Iterative string concatenation (content += new_string) in Python creates a new string every time, leading to O(N²) time complexity. "".join(list) runs in O(N).
📊 Impact: Significant reduction in time and memory allocation when generating large concatenated source files across many paths.
🔬 Measurement: Verify tests run perfectly with uv run pytest tests/io/test_get_files.py.

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PR created automatically by Jules for task 17395407827527284964 started by @georgi

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