docs: add cloud setup path via GitHub Codespaces

README.md

@@ -219,6 +219,38 @@ the container.
 
 </details>
 
+### Path 4 — GitHub Codespaces (cloud, no local GPU)
+
+Runs the same dev server in a cloud workspace, so integrated graphics and a
+local GPU never come into it. GitHub Free includes 120 core-hours a month with
+no card on file — about 60 hours on the default 2-core machine. Org and
+Enterprise plans include no free quota; current terms and quotas apply.
+
+1. Fork this repository, then open **your fork** on GitHub.
+2. Click the green **Code** button, the **Codespaces** tab, then **Create codespace on main**.
+3. In the codespace terminal:
+   ```bash
+   nvm install 24 && nvm use 24
+   npm ci
+   npm run doctor
+   GEV_ALLOWED_HOSTS="$CODESPACE_NAME-4173.$GITHUB_CODESPACES_PORT_FORWARDING_DOMAIN" npm run dev
+   ```
+4. Open the **Ports** tab, find `4173`, and click the globe icon to open it in
+   the browser. The port is private by default, so the URL asks for your GitHub
+   login.
+
+`GEV_ALLOWED_HOSTS` there is required, not optional. The dev server answers only
+the hostnames it trusts, and a forwarded port arrives on
+`$CODESPACE_NAME-4173.$GITHUB_CODESPACES_PORT_FORWARDING_DOMAIN`. Without it the
+page is `Blocked request. This host is not allowed.` Leading-dot wildcards are
+ignored on purpose, so pass that exact name.
+
+⚠️ **A codespace is reachable from the internet.** Treat it as the
+wider-publish case in [Sharing an instance](#-sharing-an-instance): leave
+Provider Settings closed to strangers, and put keys in the workspace `.env` by
+hand. `.env` is gitignored — never commit it. [SECURITY.md](SECURITY.md) has the
+threat model.
+
 ### Then power it up — in the app, not in a file
 
 Keys are upgrades, not prerequisites. When you want one, click the **POWER UP**