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/polyglot-test-agent/.github/plugin/plugin.json

@@ -7,7 +7,7 @@
   },
   "repository": "https://github.com/github/awesome-copilot",
   "license": "MIT",
-  "tags": [
+  "keywords": [
     "testing",
     "unit-tests",
     "polyglot",
@@ -19,50 +19,17 @@
     "python",
     "go"
   ],
-  "display": {
-    "ordering": "alpha",
-    "show_badge": true
-  },
-  "items": [
-    {
-      "path": "agents/polyglot-test-generator.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/polyglot-test-researcher.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/polyglot-test-planner.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/polyglot-test-implementer.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/polyglot-test-builder.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/polyglot-test-tester.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/polyglot-test-fixer.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/polyglot-test-linter.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "skills/polyglot-test-agent/unit-test-generation.prompt.md",
-      "kind": "prompt"
-    },
-    {
-      "path": "skills/polyglot-test-agent/SKILL.md",
-      "kind": "skill"
-    }
+  "agents": [
+    "./agents/polyglot-test-generator.md",
+    "./agents/polyglot-test-researcher.md",
+    "./agents/polyglot-test-planner.md",
+    "./agents/polyglot-test-implementer.md",
+    "./agents/polyglot-test-builder.md",
+    "./agents/polyglot-test-tester.md",
+    "./agents/polyglot-test-fixer.md",
+    "./agents/polyglot-test-linter.md"
+  ],
+  "skills": [
+    "./skills/polyglot-test-agent/"
   ]
 }