Finalize launch readiness docs

[bnz183]

Summary

• Updates README for PR publishing, Tiptap editor, setup detection, content QA, and AGPL license.
• Expands RELEASE_CHECKLIST with e2e, CodeQL, screenshots, PR publish smoke test, and secrets scan.
• Adds concise docs for GitHub PR publishing, editor/source mode, setup detection, and content QA.
• Strengthens CONTRIBUTING workflow (no direct main push, tests required, commit message guidance).

Test plan

• pnpm build
• pnpm test
• pnpm test:e2e
• Docs-only change — no runtime behavior changes

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