Prepare Agent Code Guard 0.3.1

[stef-k]

Summary

• bump Agent Code Guard from 0.3.0 to 0.3.1
• finalize the changelog for the shared-path/source-offset performance improvements
• include the post-merge containment and benchmark fail-closed corrections

Validation

• independent release review: READY FOR RELEASE PR at 9047988a0d1c4e80db9e02df289e76a8d8746ea3
• fresh wheel and sdist built successfully; twine check passed
• installed-wheel full suite: 326/326 passed
• wheel --version, bundled --skill-path, and doctor --json: passed
• installed-sdist parser-backed smoke: passed
• repository dogfood: expected REVIEW under --ci
• git diff --check: passed

Publication boundary

This PR prepares version 0.3.1 only. Tagging, GitHub Release creation, Trusted Publishing, public-index verification, and local pipx upgrade remain separate after merge.