[codex] Add focused example artifact checks

[stacknil]

Brief Design Summary

This PR adds focused selection support to the sbom-diff-and-risk example artifact regeneration script.

scripts/regenerate-example-artifacts.py now exposes stable artifact set slugs, --list, and repeatable --only SLUG. The default behavior remains unchanged: running the script or --check without --only still covers every configured local example artifact. The focused path helps with small maintenance updates, such as dependency example bumps, without relying on humans to remember which checked-in outputs belong to which input pair.

No runtime CLI behavior, report schema, example output content, workflows, package metadata, release tags, or publishing status changed.

Files Changed

• tools/sbom-diff-and-risk/scripts/regenerate-example-artifacts.py
• tools/sbom-diff-and-risk/tests/test_example_artifacts.py
• tools/sbom-diff-and-risk/docs/example-artifact-regeneration.md
• tools/sbom-diff-and-risk/README.md

Tests Added/Updated

Added coverage for:

• listing artifact set slugs with --list
• checking a focused subset with --check --only requirements
• preserving the existing full --check path

Validation

cd tools/sbom-diff-and-risk
python scripts/regenerate-example-artifacts.py --list
python scripts/regenerate-example-artifacts.py --check
python scripts/regenerate-example-artifacts.py --check --only requirements
python -m pytest
python -m build
$files = Get-ChildItem dist -File | ForEach-Object { $_.FullName }
python -m twine check $files
git diff --check

Results:

• list/check/focused-check script paths passed
• python -m pytest: 165 passed
• python -m build: passed, produced sbom_diff_and_risk-0.9.0 wheel and sdist
• python -m twine check: passed for wheel and sdist
• git diff --check: passed
• package version remains 0.9.0
• .github/workflows unchanged
• broad Unicode Cf/Cc scan found no non-tab/newline control or format characters in touched files

Out of Scope

• No runtime CLI behavior changes
• No report schema changes
• No example output content changes
• No workflow changes
• No package version bump
• No tag or GitHub Release
• No PyPI/TestPyPI publishing
• No production PyPI workflow