Record metadata validation evidence

[cbusillo]

Summary

• add a compact validatedThrough evidence record to .github/github.json
• point the record at the config-path migration PR and the checks used to validate the metadata
• keep the evidence scoped to metadata/config validation so expensive runtime gates remain explicit caveats where applicable

Validation

• jq shape check for non-empty validatedThrough
• codex-skills github-repo-snapshot.sh --json confirms .github/github.json and non-empty validatedThrough
• git diff --check

Part of cbusillo/codex-skills#23.

Additional local validation from commit hook:

• backend pytest: 538 passed
• CLI smoke
• frontend check/lint/unit tests/build