Ship opencoven.ai/github landing page and hosted beta waitlist

Per ROADMAP M5 and HOSTED.md landing copy.

- Landing page at opencoven.ai/github.
- Headline: "Assign it like a teammate. Get a PR back."
- Primary CTA: Join hosted beta (waitlist capture).
- Secondary CTA: Self-host the adapter.
- Buyer promise: trust continuity, familiar that knows your code/team/ship norms.

See HOSTED.md (Landing Page Copy, Buyer Promise).