docs: add Cloudflare Sandbox SDK deployment guide (#57)

* docs: add Cloudflare Sandbox SDK deployment guide

- Add docs/deploy/cloudflare.mdx with full deployment guide
- Add examples/cloudflare/ with working Worker code
- Update docs navigation to include Cloudflare option
- Update deploy index page with Cloudflare card

The example shows how to run sandbox-agent inside a Cloudflare Sandbox
with exposed ports for API access.

Co-authored-by: Shelley <shelley@exe.dev>

* fix: guard server startup to avoid port conflicts

Add health check before starting sandbox-agent to prevent 'address already
in use' errors on subsequent requests. The isServerRunning() function probes
the health endpoint to determine if setup should be skipped.

Co-authored-by: Shelley <shelley@exe.dev>

* fix: default cloudflare/sandbox:0.7.0 (latest does not exist)

* feat(cloudflare): add React frontend and improve deployment docs

- Add React + Vite frontend for Cloudflare example with sandbox-agent SDK
- Update ensureRunning to poll health endpoint instead of fixed wait
- Fix SDK fetch binding issue (globalThis.fetch.bind)
- Update docs with .dev.vars format warning and container caching tip
- Use containerFetch proxy pattern for reliable local dev

---------

Co-authored-by: Shelley <shelley@exe.dev>
Co-authored-by: Nathan Flurry <git@nathanflurry.com>
Co-authored-by: Nathan Flurry <developer@nathanflurry.com>

examples/cloudflare/src/cloudflare.ts

@@ -0,0 +1,76 @@
+import { getSandbox, type Sandbox } from "@cloudflare/sandbox";
+
+export { Sandbox } from "@cloudflare/sandbox";
+
+type Env = {
+  Bindings: {
+    Sandbox: DurableObjectNamespace<Sandbox>;
+    ASSETS: Fetcher;
+    ANTHROPIC_API_KEY?: string;
+    OPENAI_API_KEY?: string;
+  };
+};
+
+const PORT = 8000;
+
+/** Check if sandbox-agent is already running by probing its health endpoint */
+async function isServerRunning(sandbox: Sandbox): Promise<boolean> {
+  try {
+    const result = await sandbox.exec(`curl -sf http://localhost:${PORT}/v1/health`);
+    return result.success;
+  } catch {
+    return false;
+  }
+}
+
+/** Ensure sandbox-agent is running in the container */
+async function ensureRunning(sandbox: Sandbox, env: Env["Bindings"]): Promise<void> {
+  if (await isServerRunning(sandbox)) return;
+
+  // Set environment variables for agents
+  const envVars: Record<string, string> = {};
+  if (env.ANTHROPIC_API_KEY) envVars.ANTHROPIC_API_KEY = env.ANTHROPIC_API_KEY;
+  if (env.OPENAI_API_KEY) envVars.OPENAI_API_KEY = env.OPENAI_API_KEY;
+  await sandbox.setEnvVars(envVars);
+
+  // Start sandbox-agent server as background process
+  await sandbox.startProcess(`sandbox-agent server --no-token --host 0.0.0.0 --port ${PORT}`);
+
+  // Poll health endpoint until server is ready (max ~6 seconds)
+  for (let i = 0; i < 30; i++) {
+    if (await isServerRunning(sandbox)) return;
+    await new Promise((r) => setTimeout(r, 200));
+  }
+}
+
+export default {
+  async fetch(request: Request, env: Env["Bindings"]): Promise<Response> {
+    const url = new URL(request.url);
+
+    // Proxy requests to sandbox-agent: /sandbox/:name/v1/...
+    const match = url.pathname.match(/^\/sandbox\/([^/]+)(\/.*)?$/);
+    if (match) {
+      if (!env.ANTHROPIC_API_KEY && !env.OPENAI_API_KEY) {
+        return Response.json(
+          { error: "ANTHROPIC_API_KEY or OPENAI_API_KEY must be set" },
+          { status: 500 }
+        );
+      }
+
+      const name = match[1];
+      const path = match[2] || "/";
+      const sandbox = getSandbox(env.Sandbox, name);
+
+      await ensureRunning(sandbox, env);
+
+      // Proxy request to container
+      return sandbox.containerFetch(
+        new Request(`http://localhost${path}${url.search}`, request),
+        PORT
+      );
+    }
+
+    // Serve frontend assets
+    return env.ASSETS.fetch(request);
+  },
+} satisfies ExportedHandler<Env["Bindings"]>;