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/structured-autonomy/.github/plugin/plugin.json

@@ -6,5 +6,10 @@
     "name": "Awesome Copilot Community"
   },
   "repository": "https://github.com/github/awesome-copilot",
-  "license": "MIT"
+  "license": "MIT",
+  "commands": [
+    "./commands/structured-autonomy-generate.md",
+    "./commands/structured-autonomy-implement.md",
+    "./commands/structured-autonomy-plan.md"
+  ]
 }