Update CI, tests, badges, and repository links for codex-grc-engineering

[ethanolivertroy]

Context

The fork still contains references to the source repository in badges, comments, docs, and workflow comments. Initial scan found source references in README badges, docs links, and workflow comments such as OSV/Scorecard/Socket issue references.

CI also validates .claude-plugin manifests, which may need to evolve once Codex manifests/runbooks are introduced.

Tasks

• Replace repository links and badges from GRCEngClub/claude-grc-engineering to GRCEngClub/codex-grc-engineering where they refer to this fork.
• Keep attribution/source links where historically appropriate.
• Update workflow comments that close or reference source repo issues.
• Decide whether CodeRabbit/Greptile badges apply to the new repo.
• Extend tests to validate Codex docs/manifests once defined.
• Ensure link-check config does not ignore stale source-fork URLs accidentally.

Acceptance Criteria

• README badges and links reflect this repo.
• CI validates both retained Claude compatibility files and new Codex compatibility files, or documents why not.