refactor: migrate plugins to Claude Code spec format

- Move plugin manifests from .github/plugin/ to .claude-plugin/
- Convert items[] to Claude Code spec fields (agents, commands, skills)
- Rename tags to keywords, drop display/featured/instructions from plugins
- Delete all symlinks and materialized files from plugin directories
- Add eng/materialize-plugins.mjs to copy source files into plugin dirs at publish time
- Add .github/workflows/publish.yml for staged->main publishing
- Update CI triggers to target staged branch
- Update validation, creation, marketplace, and README generation scripts
- Update CONTRIBUTING.md and AGENTS.md documentation
- Include all new content from main (polyglot-test-agent, gem-browser-tester,
  fabric-lakehouse, fluentui-blazor, quasi-coder, transloadit-media-processing,
  make-repo-contribution hardening, website logo/gradient changes)

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

plugins/csharp-dotnet-development/commands/aspnet-minimal-api-openapi.md

@@ -1 +0,0 @@
-../../../prompts/aspnet-minimal-api-openapi.prompt.md

plugins/csharp-dotnet-development/commands/csharp-async.md

@@ -1 +0,0 @@
-../../../prompts/csharp-async.prompt.md

plugins/csharp-dotnet-development/commands/csharp-mstest.md

@@ -1 +0,0 @@
-../../../prompts/csharp-mstest.prompt.md

plugins/csharp-dotnet-development/commands/csharp-nunit.md

@@ -1 +0,0 @@
-../../../prompts/csharp-nunit.prompt.md

plugins/csharp-dotnet-development/commands/csharp-tunit.md

@@ -1 +0,0 @@
-../../../prompts/csharp-tunit.prompt.md

plugins/csharp-dotnet-development/commands/csharp-xunit.md

@@ -1 +0,0 @@
-../../../prompts/csharp-xunit.prompt.md

plugins/csharp-dotnet-development/commands/dotnet-best-practices.md

@@ -1 +0,0 @@
-../../../prompts/dotnet-best-practices.prompt.md

plugins/csharp-dotnet-development/commands/dotnet-upgrade.md

@@ -1 +0,0 @@
-../../../prompts/dotnet-upgrade.prompt.md