fix: downgrade headlamp-plugin SDK to 0.13.0 to fix React context errors (v0.2.2)

The plugin was built with @kinvolk/headlamp-plugin@^0.13.1, but the Headlamp server
is running with SDK version 0.13.0-alpha.11. This version mismatch caused React
context to be undefined, resulting in 'TypeError: undefined is not an object
(evaluating O2.createContext)' in the browser console.

Changes:
- Downgraded @kinvolk/headlamp-plugin from ^0.13.1 to ^0.13.0
- Removed 'main' field from package.json (carried over from v0.2.1)
- Bumped version to 0.2.2
- Created Artifact Hub metadata for 0.2.2
- Updated checksum: SHA256:3dd94e4da82a729c09eb73dcb548f89da00425169f21ff38bfb202caa442c95a

Fixes browser console error preventing plugin from loading.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

headlamp-sealed-secrets/package.json

@@ -1,6 +1,6 @@
 {
   "name": "headlamp-sealed-secrets",
-  "version": "0.2.1",
+  "version": "0.2.2",
   "description": "Headlamp plugin for Bitnami Sealed Secrets - manage encrypted Kubernetes secrets",
   "files": [
     "dist",
@@ -60,7 +60,7 @@
     "node-forge": "^1.3.1"
   },
   "devDependencies": {
-    "@kinvolk/headlamp-plugin": "^0.13.1",
+    "@kinvolk/headlamp-plugin": "^0.13.0",
     "@types/node-forge": "^1.3.11",
     "typedoc": "^0.28.16",
     "typedoc-plugin-markdown": "^4.10.0"