Add reference doc for @Observable + @MainActor interaction

[Jack-sh1]

Swift 5.9 introduced the Observation framework (@observable), but there's currently no reference covering how it interacts with Swift Concurrency — especially @mainactor.

This is a common pain point for developers, for example:

• @observable classes and @mainactor isolation
• Accessing observed properties from async contexts
• Avoiding data races when combining @observable with actors

Would you be open to adding an observation.md reference file covering these patterns? Happy to draft a PR if this fits the skill's scope.

[AvdLee]

@Jack-sh1 sounds good to me. Let's keep it focused on concurrency though, I don't want to make this skill too knowledgable on SwiftUI as well as there's a dedicated skill for that: https://github.com/AvdLee/SwiftUI-Agent-Skill