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/src/index.ts

@@ -0,0 +1,30 @@
+import { SandboxAgent } from "sandbox-agent";
+import { modal } from "sandbox-agent/modal";
+import { detectAgent } from "@sandbox-agent/example-shared";
+
+const secrets: Record<string, string> = {};
+if (process.env.ANTHROPIC_API_KEY) secrets.ANTHROPIC_API_KEY = process.env.ANTHROPIC_API_KEY;
+if (process.env.OPENAI_API_KEY) secrets.OPENAI_API_KEY = process.env.OPENAI_API_KEY;
+
+const client = await SandboxAgent.start({
+  sandbox: modal({
+    create: { secrets },
+  }),
+});
+
+console.log(`UI: ${client.inspectorUrl}`);
+
+const session = await client.createSession({
+  agent: detectAgent(),
+});
+
+session.onEvent((event) => {
+  console.log(`[${event.sender}]`, JSON.stringify(event.payload));
+});
+
+session.prompt([{ type: "text", text: "Say hello from Modal in one sentence." }]);
+
+process.once("SIGINT", async () => {
+  await client.destroySandbox();
+  process.exit(0);
+});