WinUI plugin enhancements and add MVVM Toolkit skill

[alvinashcraft]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction, prompt, agent, skill, or workflow file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, skill, or workflow with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.
• I am targeting the staged branch for this pull request.

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Description

Adding a new MVVM Toolkit skill with some additional reference files to add details to different aspects of its usage. I also updated the WinUI 3 plugin to include the new skill plus the existing MS Store CLI and winapp CLI skills.

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• Update to existing instruction, prompt, agent, plugin, skill, or workflow.
• Other (please specify):

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Additional Notes

I would like a review from @niels9001 since he's the original author of the WinUI plugin. I don't want to create too much bloat in that plugin and have it not function as originally intended for UWP migrations.

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[github-actions]

🔍 Skill Validator Results

⚠️ Warnings or advisories found

Scope	Checked
Skills	3
Agents	1
Total	4
Severity	Count
---	---:
❌ Errors	0
⚠️ Warnings	1
ℹ️ Advisories	0

Summary

Level	Finding
ℹ️	Found 3 skill(s)
ℹ️	[mvvm-toolkit] 📊 mvvm-toolkit: 2,553 BPE tokens [chars/4: 2,696] (standard ~), 14 sections, 9 code blocks
ℹ️	[mvvm-toolkit] ⚠ Skill is 2,553 BPE tokens (chars/4 estimate: 2,696) — approaching "comprehensive" range where gains diminish.
ℹ️	[mvvm-toolkit-di] 📊 mvvm-toolkit-di: 1,998 BPE tokens [chars/4: 2,339] (detailed ✓), 13 sections, 10 code blocks
ℹ️	[mvvm-toolkit-messenger] 📊 mvvm-toolkit-messenger: 1,946 BPE tokens [chars/4: 2,214] (detailed ✓), 17 sections, 14 code blocks
ℹ️	✅ All checks passed (3 skill(s))

Full validator output

```text Found 3 skill(s) [mvvm-toolkit] 📊 mvvm-toolkit: 2,553 BPE tokens [chars/4: 2,696] (standard ~), 14 sections, 9 code blocks [mvvm-toolkit] ⚠ Skill is 2,553 BPE tokens (chars/4 estimate: 2,696) — approaching "comprehensive" range where gains diminish. [mvvm-toolkit-di] 📊 mvvm-toolkit-di: 1,998 BPE tokens [chars/4: 2,339] (detailed ✓), 13 sections, 10 code blocks [mvvm-toolkit-messenger] 📊 mvvm-toolkit-messenger: 1,946 BPE tokens [chars/4: 2,214] (detailed ✓), 17 sections, 14 code blocks ✅ All checks passed (3 skill(s)) ```

[Copilot]

Pull request overview

Adds an MVVM Toolkit skill and integrates it into the existing winui3-development plugin so WinUI 3 developers can reference MVVM patterns alongside packaging and Store publishing workflows.

Changes:

• Added new mvvm-toolkit skill with focused reference pages (generators, commands, messenger, DI, validation, troubleshooting, walkthrough).
• Added a new mvvm-toolkit instruction file for C#/XAML/.csproj guidance when using CommunityToolkit.Mvvm.
• Expanded the winui3-development plugin metadata/docs to include mvvm-toolkit, winapp-cli, and msstore-cli, and updated marketplace/docs indices accordingly.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
skills/mvvm-toolkit/SKILL.md	New MVVM Toolkit skill entrypoint and high-level guidance with links to bundled references.
skills/mvvm-toolkit/references/source-generators.md	Reference for MVVM Toolkit source generator attributes and generated code behavior.
skills/mvvm-toolkit/references/relaycommand-cookbook.md	Recipes for [RelayCommand], RelayCommand, and AsyncRelayCommand usage.
skills/mvvm-toolkit/references/messenger-patterns.md	Messenger usage patterns (implementations, channels, request/reply, lifetimes).
skills/mvvm-toolkit/references/dependency-injection.md	Guidance for using Microsoft.Extensions.DependencyInjection with MVVM Toolkit.
skills/mvvm-toolkit/references/validation.md	Validation guidance for ObservableValidator and DataAnnotations.
skills/mvvm-toolkit/references/troubleshooting.md	Common generator diagnostics and common MVVM Toolkit troubleshooting items.
skills/mvvm-toolkit/references/end-to-end-walkthrough.md	End-to-end WinUI 3 walkthrough tying together MVVM Toolkit + DI + messaging.
instructions/mvvm-toolkit.instructions.md	New instructions file defining MVVM Toolkit conventions across C#/XAML projects.
plugins/winui3-development/README.md	Updated plugin README to document the expanded scope and new slash commands.
plugins/winui3-development/.github/plugin/plugin.json	Updated plugin metadata, tags, version, and included skills list.
docs/README.skills.md	Added mvvm-toolkit to the skills index.
docs/README.instructions.md	Added mvvm-toolkit instructions to the instructions index.
docs/README.plugins.md	Updated winui3-development plugin listing (description, item count, tags).
.github/plugin/marketplace.json	Updated marketplace entry for winui3-development description/version.

[aaronpowell]

As the validator notes, there is a risk on the mvvm-toolkit still being large, but I see that you've already done an additional pass on it to condense it down and I don't think without splitting it (which is arguably detrimental) you can achieve much improvement on what's there.

I'll give it a few days to see if the original plugin author replies, but generally speaking it is good to go.