docs(integrations): add CopilotKit integration page

[mme]

Summary

Adds an integration page for CopilotKit to the catalog at docs/integrations/copilotkit.md, with its icon in docs/integrations/assets/.

CopilotKit is an open source set of frontend libraries and a runtime that connect applications to agents over AG-UI. With ADK, the ag-ui-adk package exposes an agent as an AG-UI endpoint, and CopilotKit gives that endpoint a chat surface, frontend tools, generative UI, and human-in-the-loop controls in React, Angular, Vue, React Native, or Slack.

What the page covers

• Use cases: chat, frontend tools, generative UI, human-in-the-loop, and messaging channels
• Prerequisites and installation: Python and npm packages
• Use with agent: a four-step walkthrough that exposes an ADK agent over AG-UI, registers it with CopilotKit Runtime, renders chat in React, and adds a browser-side tool the agent can call
• Available hooks: a table of the React hooks and what each one does
• Other clients: Angular, Vue, and React Native, which share the same runtime route and endpoint
• Messaging channels: connecting a Slack workspace to the same endpoint with the open source Channels SDK
• Additional resources

The existing AG-UI page covers the protocol and a create quickstart for new projects. This page links to it and focuses on adding CopilotKit to an existing ADK project, so the two do not overlap.

Notes for reviewers

• catalog_tags is left out because none of the existing tags describe a frontend integration, and the AG-UI and A2UI cards carry none either. Happy to add one if you have a preference.
• Model strings use the gemini-flash-latest alias, the agent variable is root_agent, and internal links are site-relative.

Verification

• mkdocs build --strict passes and the card renders in the catalog
• The Python sample was run against the published google-adk and ag-ui-adk packages; the endpoint starts and streams RUN_STARTED for the documented request
• Every TypeScript import and option was checked against the type declarations of the published CopilotKit packages
• All external links resolve

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