Add Modal and ComputeSDK built-in providers, update examples and docs

- Add `sandbox-agent/modal` provider using Modal SDK with node:22-slim image
- Add `sandbox-agent/computesdk` provider using ComputeSDK's unified sandbox API
- Update Modal and ComputeSDK examples to use new SDK providers
- Update Modal and ComputeSDK deploy docs with provider-based examples
- Add Modal to quickstart CodeGroup and docs.json navigation
- Add provider test entries for Modal and ComputeSDK
- Remove old standalone example files (modal.ts, computesdk.ts)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

examples/modal/package.json

@@ -3,7 +3,7 @@
   "private": true,
   "type": "module",
   "scripts": {
-    "start": "tsx src/modal.ts",
+    "start": "tsx src/index.ts",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {