Reorganize plugins into domain-specific groupings (#274)

* Reorganize plugins per agreed structure (#271)

Split the dotnet plugin into domain-specific plugins per discussion
in #271:

- dotnet: narrowed to common everyday C#/.NET coding tasks
- dotnet-diag: perf investigations, debugging, incident analysis
- dotnet-data: data access and Entity Framework
- dotnet-aspnet, dotnet-wpf, dotnet-winforms, dotnet-maui: scaffolded
  empty to convey intent for future framework-specific skills

Moved skills, tests, and CODEOWNERS entries accordingly.
No file content was changed; all skill/test moves are pure renames.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Improve dotnet-diag and dotnet-winforms descriptions

- dotnet-diag: add '.NET' qualifier to clarify scope
- dotnet-winforms: use official 'Windows Forms (WinForms)' name

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add .NET qualifier to dotnet-data description

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove empty plugins; list them in CONTRIBUTING.md as future options

Remove the four empty placeholder plugins (dotnet-aspnet, dotnet-wpf,
dotnet-winforms, dotnet-maui) that had only plugin.json and no skills.

Add a 'Plugin organization' section to CONTRIBUTING.md that describes the
domain-specific plugin groupings introduced in this PR and lists the four
removed plugin names as reserved candidates for future contributions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Create dotnet-upgrade plugin with migration/upgrade skills

Move migrate-nullable-references, thread-abort-migration, and
dotnet-aot-compat from dotnet to the new dotnet-upgrade plugin.

Update CODEOWNERS paths (same owners), CONTRIBUTING.md plugin table,
and fix the thread-abort-migration SKILL.md link in CONTRIBUTING.md.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

plugins/dotnet-diag/plugin.json

@@ -0,0 +1,7 @@
+{
+  "name": "dotnet-diag",
+  "version": "0.1.0",
+  "description": "Skills for .NET performance investigations, debugging, and incident analysis.",
+  "skills": "./skills/",
+  "agents": "./agents/"
+}