Add Codex docs sync on PR merge

[leenk7991]

What

Adds .github/workflows/update_docs.yml: on merge to main, opens a PR against the docs repo describing the change. Calls Corgea/internal_actions/sync-docs-action@main unmodified, the same pattern our other repos already use. First workflow in this repo.

Notes

• Requires OPENAI_API_KEY and DOCS_REPO_TOKEN to be configured. A preflight step warns and skips when either is absent, so this is safe to merge now and begins working once they are set. The secrets context is not available to a job-level if, hence the step.
• Fork PRs receive no secrets on pull_request, so the job skips them rather than showing a failed check.
• No actions/checkout step: the action checks this repo out to ./source itself, and a root checkout would place SKILL.md files under the model's working directory as a decoy edit target.
• Docs guidance rides on the existing pr_info input, so the shared action needed no change.

Merge order: third, after Corgea/docs#283 so the first sync has a page to update.

Related: Corgea/docs#283, Corgea/internal_actions#2