feat(agentkit): migrate Stagehand example to v3 API (#58)

* feat(agentkit): migrate Stagehand example to v3 API

- Delegate act/observe/extract to Stagehand instance; navigate via context.pages
- Bump @browserbasehq/stagehand to ^3.2.0 and align AgentKit State/AgentResult
- Add .env.example for required keys

Made-with: Cursor

* Remove 'anthropic/claude-haiku-3-5' from models

Removed 'anthropic/claude-haiku-3-5' from the model list.

Author: Kylejeong2