GDK support for buzz

In order for https://github.com/block/buzz/pull/3262 to adopt GDK crates only (and not link against the `goose` app crate), GDK crates will need:

- openrouter provider
- PromptManager (maybe? consider what's useful here vs what should be app-implemented)
- skills filesystem discovery
- [more items as needed]

The issue references GDK crates and a linked pull request in the buzz repository. Start by examining the buzz PR #3262 to understand the context and the specific GDK dependencies. Review the existing GDK crate structure in the goose repository to identify where to add openrouter provider support, PromptManager integration, and skills filesystem discovery. Look for similar provider implementations and skill discovery patterns in the codebase as a reference. Testing will involve ensuring the GDK crates can function independently without linking to the main goose app crate.

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