[UM-CLONE-2] Stagehand — observe()/extract()/act() typed API

[Delqhi]

Source

• Project: Stagehand
• License: MIT
• Clone mode: code

Feature we want

Three-verb agent surface: observe(intent) returns candidate actions, extract(zodSchema) returns typed structured data, act(intent) performs an action. All driven by an LLM under the hood.

Why we want it

This is the de-facto API shape the market expects. Closes existing issues #4/#5/#6.

Acceptance criteria

• Public API: await page.observe(intent), await page.extract(schema), await page.act(intent)
• Zod schema validation on extract; typed return inferred from schema
• Configurable model provider (OpenAI/Anthropic/Vercel AI Gateway) via DI
• Mock-LLM mode for unit tests
• Examples in examples/stagehand-style/
• PR body cites source repo + commit SHA + license
• THIRD_PARTY_NOTICES.md updated
• Parity test added (proves our clone behaves like source for the relevant case)
• CHANGELOG entry: cloned from Stagehand@<sha>

Reference

See COMPETITIVE_STRATEGY.md for the full doctrine.