Update the init template to use Stagehand

[akeimach]

We can use Stagehand as our starter template now that we support secrets!

Testing:

1. [in the sdk-functions-node repo] pnpm build && pnpm pack
2. alias bb-dev="node $HOME/dev/sdk-functions-node/dist/cli.js" to create your local version of bb
3. cd ~/dev && bb-dev init sdk-test && cd sdk-test. Check that index.ts is the inline Stagehand starter, that pnpm-workspace.yaml has only allowBuilds, that package.json has packageManager and no devEngines, and the printed instructions share how to upload the extension id
4. cp ~/dev/sdk-functions-node/browserbasehq-sdk-functions-*.tgz . && pnpm add ./browserbasehq-sdk-functions-1.0.2.tgz. package.json should now show "@browserbasehq/sdk-functions": "file:browserbasehq-sdk-functions-1.0.2.tgz".
5. browse cloud extensions upload node_modules/@browserbasehq/stagehand/dist/assets/stagehand-extension.zip,, then replace your-extension-id in index.ts
6. Put your prod key in .env as BROWSERBASE_API_KEY=<your prod key> and also run export BROWSERBASE_API_KEY=<your prod key>
7. browse cloud secrets create BROWSERBASE_API_KEY --env BROWSERBASE_API_KEY
8. pnpm bb dev index.ts, then in a new terminal:
   curl -X POST http://127.0.0.1:14113/v1/functions/my-function/invoke -H 'Content-Type: application/json' --data '{}'
9. pnpm bb publish index.ts
10. browse functions secrets attach <functionId> <secretId>
11. pnpm bb invoke <functionId> and make sure you get the same result as step 8.

[Kylejeong2]

looks good, i assume you tested locally

only note is to maybe use model gateway instead