Keystone of the GitHub integration: lets admins paste any GitHub issue or PR URL onto a Hollahoop post. Parses owner/repo/number, validates the org has the GitHub App installed on that repo, stores the link in a new github_post_links table (post_id PK, repo_id, issue_number, etag, last_synced). Renders an "acme/api#123" pill on the post detail page and on roadmap cards. Optional: parse cross-repo refs ("acme/api#123") in post bodies and offer "Convert to link?" suggestion.

This is the cheapest single feature to ship and seeds the data every other GitHub event handler depends on. Build before auto-ship and release-driven changelog drafts.