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/rug-agentic-workflow/.github/plugin/plugin.json

@@ -7,26 +7,16 @@
   },
   "repository": "https://github.com/github/awesome-copilot",
   "license": "MIT",
-  "tags": [
+  "keywords": [
     "agentic-workflow",
     "orchestration",
     "subagents",
     "software-engineering",
     "qa"
   ],
-  "items": [
-    {
-      "path": "agents/rug-orchestrator.agent.md",
-      "kind": "agent",
-      "usage": "recommended\n\nThis workflow is designed as a coordinated trio:\n- `RUG` orchestrates decomposition, delegation, and validation loops.\n- `SWE` executes implementation tasks.\n- `QA` validates requirements, edge cases, and regressions.\n\nTypical usage:\n1. Start with `RUG` for multi-step tasks.\n2. Let `RUG` delegate coding to `SWE`.\n3. Let `RUG` delegate verification to `QA`.\n\nBest results:\n- Keep each delegated task narrowly scoped.\n- Require explicit acceptance criteria per task.\n- Run a final cross-task validation pass before completion."
-    },
-    {
-      "path": "agents/swe-subagent.agent.md",
-      "kind": "agent"
-    },
-    {
-      "path": "agents/qa-subagent.agent.md",
-      "kind": "agent"
-    }
+  "agents": [
+    "./agents/rug-orchestrator.md",
+    "./agents/swe-subagent.md",
+    "./agents/qa-subagent.md"
   ]
 }