feat: add Dockerfile, Gitea Actions CI, and Artifact Hub metadata

Multi-stage Dockerfile produces a lightweight image with the compiled
plugin at /plugins/. Gitea Actions workflow triggers on tag push (v*),
builds the plugin, pushes to the Gitea container registry, and creates
a release with the tarball attached.

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

package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "start": "headlamp-plugin start",
     "build": "headlamp-plugin build",
+    "package": "headlamp-plugin package",
     "tsc": "tsc --noEmit"
   },
   "devDependencies": {