feat: add Artifact Hub metadata and GitHub Actions release workflow

Artifact Hub requires a GitHub-hosted repo for Headlamp plugins.
Since Gitea push-mirrors git objects but not releases, a GitHub
Actions workflow builds and publishes GitHub Releases with the
tarball that Artifact Hub needs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -0,0 +1,37 @@
+name: GitHub Release
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+permissions:
+  contents: write
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build plugin
+        run: npx @kinvolk/headlamp-plugin build
+
+      - name: Package tarball
+        run: npx @kinvolk/headlamp-plugin package
+
+      - name: Create GitHub Release
+        uses: softprops/action-gh-release@v2
+        with:
+          files: "*.tar.gz"
+          generate_release_notes: true