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/README.md

@@ -0,0 +1,46 @@
+# Cloudflare Sandbox Agent Example
+
+Deploy sandbox-agent inside a Cloudflare Sandbox.
+
+## Prerequisites
+
+- Cloudflare account with Workers Paid plan
+- Docker running locally for `wrangler dev`
+- `ANTHROPIC_API_KEY` or `OPENAI_API_KEY` for the coding agents
+
+## Setup
+
+1. Install dependencies:
+
+```bash
+pnpm install
+```
+
+2. Create `.dev.vars` with your API keys:
+
+```bash
+echo "ANTHROPIC_API_KEY=your-api-key" > .dev.vars
+```
+
+## Development
+
+Start the development server:
+
+```bash
+pnpm run dev
+```
+
+Test the endpoint:
+
+```bash
+curl http://localhost:8787
+```
+
+## Deploy
+
+```bash
+pnpm run deploy
+```
+
+Note: Production preview URLs require a custom domain with wildcard DNS routing.
+See [Cloudflare Production Deployment](https://developers.cloudflare.com/sandbox/guides/production-deployment/) for details.