Docs: CONTRIBUTING note — fork PRs skip Agentic PR review check

[leo-aa88]

Context

The bundled .github/workflows/agentctl-pr-review.yml review job only runs when:

github.event.pull_request.head.repo.full_name == github.repository

Fork-originated PRs therefore get no Agentic PR review check (intentional: no OPENAI_API_KEY / same-repo secrets). External contributors may not realize automated review exists until a maintainer works from an in-repo branch.

docs/GITHUB_ACTIONS.md already explains fork PRs and secrets.

Proposal

Add CONTRIBUTING.md (or a short section in an existing contributor doc) that states:

• First-time / fork contributors may not see the Agentic PR review workflow on their PR.
• Maintainers can re-run or validate from a branch on the canonical repo when appropriate.

Acceptance criteria

• New contributors discover the behavior without reading Actions internals.
• Link to docs/GITHUB_ACTIONS.md for full fork + secret details.