Move repo workflow metadata to github.json

[cbusillo]

Summary

• rename .github/github-repo-workflow.json to .github/github.json
• update repo docs/agent instructions to point at the new canonical metadata path
• leave existing metadata contents unchanged for the validation-evidence follow-up in Fill validatedThrough metadata for audited repos codex-skills#23

Validation

• jq empty .github/github.json
• confirmed .github/github-repo-workflow.json no longer exists
• old filename reference scan with rg "github-repo-workflow\.json"
• codex-skills github-repo-snapshot.sh --json reports .github/github.json

Part of cbusillo/codex-skills#24.