Add an agent migration skill for Project Resource v2

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

Project Resource v2 changes how C# projects are represented and run so Aspire can stop serially waiting for project builds. Existing applications need a reliable path from the current `ProjectResource` model to the preview APIs; without targeted migration guidance, users and coding agents must infer the required changes and can easily lose references, endpoints, arguments, environment variables, or build behavior.

### Describe the solution you'd like

Add an Aspire agent skill that recognizes AppHosts using the current project-resource APIs and guides or performs migration to the Project Resource v2 preview.

The skill should:

- Detect project resources and related builder customizations that need migration.
- Convert supported patterns to the Project Resource v2 APIs while preserving resource names, references, endpoints, arguments, environment variables, and wait relationships.
- Explain unsupported or ambiguous patterns instead of silently producing a behavior change.
- Validate the resulting AppHost and summarize any manual follow-up required.
- Document that Project Resource v2 is a preview and identify its current limitations.

### Additional context

This is the migration portion of the 13.6 roadmap item nested under **Project Resource v2 / Stop waiting on builds**. The parent architecture is tracked by #16386. This issue is intentionally focused on the user migration experience rather than the underlying Project Resource v2 implementation.