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/tsconfig.json

@@ -0,0 +1,15 @@
+{
+  "compilerOptions": {
+    "target": "esnext",
+    "lib": ["esnext"],
+    "module": "esnext",
+    "moduleResolution": "bundler",
+    "types": ["@cloudflare/workers-types"],
+    "esModuleInterop": true,
+    "forceConsistentCasingInFileNames": true,
+    "strict": true,
+    "skipLibCheck": true,
+    "noEmit": true
+  },
+  "include": ["src/**/*.ts"]
+}