docs: add usage and integration guide

[GeneralJerel]

Summary

• Adds a /docs directory with 10 markdown files covering how to use the app and how to bring these patterns into your own project
• Covers: getting started, architecture, agent state, generative UI, agent tools, human-in-the-loop, MCP integration, deployment, and a step-by-step migration guide
• Code examples reference actual source files in the repo

Test plan

• Verify all markdown files render correctly on GitHub
• Confirm internal links between docs resolve properly
• Check code examples match current source

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