fix(ckg): Accept file paths

[Gujiassh]

Summary

• allow the CKG tool path argument to point at a file inside the target codebase
• resolve file paths to the git repository root when available, otherwise fall back to the file's parent directory
• clarify the path parameter description and add regression tests

Closes #354

Validation

• uv run ruff check trae_agent/tools/ckg_tool.py tests/tools/test_ckg_tool.py
• PYTHONPATH=. uv run --extra test --extra evaluation pytest tests/tools/test_ckg_tool.py -q