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/copilot-sdk/.github/plugin/plugin.json

@@ -7,7 +7,7 @@
   },
   "repository": "https://github.com/github/awesome-copilot",
   "license": "MIT",
-  "tags": [
+  "keywords": [
     "copilot-sdk",
     "sdk",
     "csharp",
@@ -18,31 +18,7 @@
     "ai",
     "github-copilot"
   ],
-  "featured": true,
-  "display": {
-    "ordering": "manual",
-    "show_badge": true
-  },
-  "items": [
-    {
-      "path": "instructions/copilot-sdk-csharp.instructions.md",
-      "kind": "instruction"
-    },
-    {
-      "path": "instructions/copilot-sdk-go.instructions.md",
-      "kind": "instruction"
-    },
-    {
-      "path": "instructions/copilot-sdk-nodejs.instructions.md",
-      "kind": "instruction"
-    },
-    {
-      "path": "instructions/copilot-sdk-python.instructions.md",
-      "kind": "instruction"
-    },
-    {
-      "path": "skills/copilot-sdk/SKILL.md",
-      "kind": "skill"
-    }
+  "skills": [
+    "./skills/copilot-sdk/"
   ]
 }