winui-packaging: ship a tested GitHub Actions workflow file instead of an inline YAML snippet

[nmetulev]

97: #97

Problem

From Joe's blog post:

CI was next. The workflow the agent generated didn't work out of the box, so I just removed it.

The current winui-packaging skill embeds an example GitHub Actions YAML snippet as inline markdown (lines 67–90). The snippet has at least these problems:

1. Hard-codes ./bin/x64/Release/ — actual output path varies by TargetFramework, Configuration, and RID, and is usually bin/x64/Release/<tfm>/win-x64/ or similar after RuntimeIdentifiers is set.
2. Doesn't set up the .NET SDK before dotnet build (relies on whatever ships on the runner).
3. Single-platform only — fails on Arm64.
4. Never exercised by our own CI, so we have no signal when it drifts.

The result: the agent emits a workflow that looks right and doesn't run. Pattern is the deeper issue — every prose-embedded snippet we tell the agent to copy is unverified.

Proposed fix

• Move the YAML to a real file: plugins/winui/skills/winui-packaging/references/github-actions/build-and-package.yml.
• Make it a proper matrix workflow (x64, arm64), with actions/setup-dotnet, correct artifact globs, signing from a repo secret (and a graceful fallback to ephemeral dev cert for non-secret-bearing forks / first-time setup).
• Exercise it against a sample WinUI app inside this repo's CI so any breakage is caught here, not by users.
• Skill body instructs the agent to copy the file in (Copy-Item from the installed skill path, or gh repo clone of just that path) rather than emit YAML from scratch.

Acceptance criteria

• Workflow lives as a real file under references/github-actions/.
• CI in this repo runs it against a sample WinUI 3 app on every PR.
• Skill instructs the agent to copy the file rather than regenerate it.
• Both x64 and arm64 produce uploaded .msix artifacts on each run.
• Inline YAML in SKILL.md is removed (or reduced to a 5-line "what it does" excerpt with a pointer to the real file).

Acknowledgments

Identified by Joe Finney in Building Fluent Icon Finder with the WinUI Copilot skill.