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/wrangler.jsonc

@@ -0,0 +1,33 @@
+{
+  "$schema": "node_modules/wrangler/config-schema.json",
+  "name": "sandbox-agent-cloudflare",
+  "main": "src/cloudflare.ts",
+  "compatibility_date": "2025-01-01",
+  "compatibility_flags": ["nodejs_compat"],
+  "assets": {
+    "directory": "./dist",
+    "binding": "ASSETS"
+  },
+  "containers": [
+    {
+      "class_name": "Sandbox",
+      "image": "./Dockerfile",
+      "instance_type": "lite",
+      "max_instances": 1
+    }
+  ],
+  "durable_objects": {
+    "bindings": [
+      {
+        "class_name": "Sandbox",
+        "name": "Sandbox"
+      }
+    ]
+  },
+  "migrations": [
+    {
+      "new_sqlite_classes": ["Sandbox"],
+      "tag": "v1"
+    }
+  ]
+}