[Skill] WinUI runtime error diagnostics

Parent: #126

Summary

A guidance skill that teaches an agent to diagnose the most common WinUI 3 / Windows App SDK runtime errors — symptom → root cause → fix — so it stops guessing. Complements the WinDbg skill for dump cases.

Target error classes (most-common, verified)

• COMException 0x80040154 (REGDB_E_CLASSNOTREG, "Class not registered") — usually missing package identity or Windows App SDK runtime.
• Wrong-thread marshalling — RPC_E_WRONG_THREAD (0x8001010E) — touching UI off the DispatcherQueue; guide to DispatcherQueue.TryEnqueue.
• Bootstrapper / runtime-not-found — unpackaged apps missing Bootstrap.Initialize() / mismatched runtime version.
• XAML parse / binding errors — x:Bind failures, missing StaticResource, XamlParseException.

What the skill should teach the agent

• How to recognize each symptom from build/run logs and error text.
• How to decode HRESULT / NTSTATUS / GetLastError (facility/severity bits) to meaning.
• A checklist + concrete fix per class (packaged vs unpackaged, dispatcher marshalling, runtime install, XAML resource resolution).

Deliverables

• Skill (SKILL.md + supporting guidance files) under plugins/.
• Worked examples for each error class.

References

• Windows App SDK known issues; packaged vs unpackaged deployment docs.

Start by reviewing the Windows App SDK known-issues material and the packaged versus unpackaged deployment docs referenced in the issue. Add the skill under plugins/, including SKILL.md, supporting guidance, and worked examples for each listed error class. Done means the guidance covers symptom recognition, cause, decoding, and a concrete fix for all four classes.