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>

.github/workflows/check-line-endings.yml

@@ -2,9 +2,9 @@ name: Check Line Endings
 
 on:
   push:
-    branches: [main]
+    branches: [staged]
   pull_request:
-    branches: [main]
+    branches: [staged]
 
 permissions:
   contents: read

.github/workflows/codespell.yml

@@ -2,9 +2,9 @@ name: Check Spelling
 
 on:
   push:
-    branches: [main]
+    branches: [staged]
   pull_request:
-    branches: [main]
+    branches: [staged]
 
 permissions:
   contents: read

.github/workflows/publish.yml

@@ -0,0 +1,49 @@
+name: Publish to main
+
+on:
+  push:
+    branches: [staged]
+
+permissions:
+  contents: write
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout staged branch
+        uses: actions/checkout@v4
+        with:
+          ref: staged
+          fetch-depth: 0
+
+      - name: Extract Node version from package.json
+        id: node-version
+        run: |
+          NODE_VERSION=$(jq -r '.engines.node // "22"' package.json)
+          echo "version=${NODE_VERSION}" >> "$GITHUB_OUTPUT"
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ steps.node-version.outputs.version }}
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Materialize plugin files
+        run: node eng/materialize-plugins.mjs
+
+      - name: Build generated files
+        run: npm run build
+
+      - name: Fix line endings
+        run: bash scripts/fix-line-endings.sh
+
+      - name: Publish to main
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git add -A
+          git commit -m "chore: publish from staged [skip ci]" --allow-empty
+          git push origin HEAD:main --force

.github/workflows/validate-readme.yml

@@ -2,6 +2,7 @@ name: Validate README.md
 
 on:
   pull_request:
+    branches: [staged]
     types: [opened, synchronize, reopened]
     paths:
       - "instructions/**"