Onboard OpenCode to Demofy

The generic path the spec reserves for "other agents" — there is no skills repo to install, so it is the same web-app instructions in OpenCode’s config shape.

Commands

There is no `npx -y skills add` command here — Demofy publishes no skills
repository and no MCP endpoint. Add nothing to `opencode.json`; instead:

1. Read [`https://demofy.io/llms.txt`](https://demofy.io/llms.txt).
2. To make a demo: [https://demofy.io/](https://demofy.io/) — a URL and one line of intent, no account required.
3. To sign up: [https://demofy.io/sign-up](https://demofy.io/sign-up).
4. For one-click capture: install the PWA at [https://demofy.io/](https://demofy.io/), or build `extension/` from source with `wxt` (unpublished, open source).