feat(marketplace): add npm packages, revert to git URL for local use

Published @lthn/core-agent on npm. External plugins use git URLs
locally, npm for public distribution.

Co-Authored-By: Virgil <virgil@lethean.io>

.claude-plugin/package.json

@@ -0,0 +1,21 @@
+{
+  "name": "@lthn/core-agent",
+  "version": "0.2.0",
+  "description": "Host UK Claude Code plugin marketplace — development hooks, code review, and verification",
+  "license": "EUPL-1.2",
+  "author": {
+    "name": "Host UK",
+    "email": "hello@host.uk.com"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://forge.lthn.ai/core/agent.git",
+    "directory": ".claude-plugin"
+  },
+  "keywords": [
+    "claude-code-marketplace",
+    "claude-code-plugin",
+    "development",
+    "code-review"
+  ]
+}