Add two skills to the genui package

[redbrogdon]

This PR introduces two new AI agent skills for the GenUI package, along with a brief update to the README.md.

Specifically, this adds:

• integrate-genui-firebase: Guides AI agents on how to integrate the GenUI package into a Flutter app and set up a basic conversational agent using Firebase AI Logic, covering everything from initialization to surface rendering.
• create-catalog-item Provides instructions for AI agents on how to create a new CatalogItem widget, including defining the data schema, widget builder, action resolution, and handling imports.
• README update: Adds a short blurb to the root README.md to introduce or document these new capabilities.

Both of these have been evaluated manually, but I don't yet have a dataset for doing automated evals for them.

Pre-launch Checklist

• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I read the [Contributors Guide].
• [N/A] I have added sample code updates to the [changelog].
• [N/A] I updated/added relevant documentation (doc comments with ///).

[gemini-code-assist]

Code Review

This pull request introduces two new AI agent skills and updates the README. The new skills provide detailed instructions for an AI, and my review focuses on improving the robustness and correctness of the code examples within these skills. I've suggested changes to make the code safer and more idiomatic, particularly around error handling and event processing. Additionally, I've noted a URL in the README that appears to point to a personal fork instead of the canonical repository.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[gspencergoog]

Looks like there are copyright issues (which are not your fault). I'd try syncing to head and see if they go away.