Allow safe relative subdirectory paths in GraphML node source labels

[Titas-Ghosh]

Hi @pradeeban , This PR fixes #297 ,

Summary

This PR allows GraphML node source labels to use relative subpaths (for example, script.py while preserving path-safety checks. Also this PR contains only actual functional code changes (no formatting-only/EOL-only churn) and is limited to 2 files: mkconcore.py and tests/test_cli.py.

Problem

mkconcore.py currently rejects / and \ in node source labels, which forces a flat src/ layout.
That makes larger workflows harder to organize and maintain.

What Changed

• Added safe_relpath() validation for node source paths.
• Allowed relative subpaths in node labels.
• Continued to reject unsafe paths:
• path traversal (..)
• absolute paths (/..., ~...)
• drive-root style paths (C:...)
• invalid empty segments
• Ensured parent directories are created before writing/copying generated artifacts tied to subpath-based sources.
• Added CLI integration test coverage for subdirectory source workflow execution.

Files Changed

mkconcore.py
test_cli.py