Harden content integrity signature checks

[fallintoplace]

Summary

• Walk signed skill directories and fail when a file is neither signed nor explicitly ignored by signature metadata.
• Validate signed resource names before reading files: reject absolute paths, .., non-normalized names, and paths that resolve outside the skill directory.
• Honor serialization.allow_symlinks only for symlinks that stay inside the skill directory.

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m py_compile .github/scripts/verify_content_integrity.py
• git diff --check
• Synthetic temp-dir checks for unsigned extras, ignored files, absolute paths, .., non-normalized paths, disallowed symlinks, allowed in-root symlinks, and symlinks escaping the skill directory.
• python3 .github/scripts/verify_content_integrity.py fails as expected on current catalog drift, now including skills/physical-ai-neural-reconstruction/evals/evals.json: UNSIGNED EXTRA.