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/testing-automation/.github/plugin/plugin.json

@@ -7,7 +7,7 @@
   },
   "repository": "https://github.com/github/awesome-copilot",
   "license": "MIT",
-  "tags": [
+  "keywords": [
     "testing",
     "tdd",
     "automation",
@@ -17,54 +17,17 @@
     "jest",
     "nunit"
   ],
-  "display": {
-    "ordering": "alpha",
-    "show_badge": true
-  },
-  "items": [
-    {
-      "path": "agents/tdd-red.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/tdd-green.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/tdd-refactor.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/playwright-tester.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "instructions/playwright-typescript.instructions.md",
-      "kind": "instruction"
-    },
-    {
-      "path": "instructions/playwright-python.instructions.md",
-      "kind": "instruction"
-    },
-    {
-      "path": "prompts/playwright-explore-website.prompt.md",
-      "kind": "prompt"
-    },
-    {
-      "path": "prompts/playwright-generate-test.prompt.md",
-      "kind": "prompt"
-    },
-    {
-      "path": "prompts/csharp-nunit.prompt.md",
-      "kind": "prompt"
-    },
-    {
-      "path": "prompts/java-junit.prompt.md",
-      "kind": "prompt"
-    },
-    {
-      "path": "prompts/ai-prompt-engineering-safety-review.prompt.md",
-      "kind": "prompt"
-    }
+  "agents": [
+    "./agents/tdd-red.md",
+    "./agents/tdd-green.md",
+    "./agents/tdd-refactor.md",
+    "./agents/playwright-tester.md"
+  ],
+  "commands": [
+    "./commands/playwright-explore-website.md",
+    "./commands/playwright-generate-test.md",
+    "./commands/csharp-nunit.md",
+    "./commands/java-junit.md",
+    "./commands/ai-prompt-engineering-safety-review.md"
   ]
 }