feat(plugin): publish as Claude Code marketplace plugin

Update .claude-plugin/marketplace.json and plugin.json to v2.9.5 with
full metadata. Add plugin install instructions to README as the
recommended install method. The repo root serves as both the marketplace
and the plugin - skills/last30days/SKILL.md (symlink) is discovered
automatically.

Users can now install with:
  /plugin marketplace add mvanhorn/last30days-skill
  /plugin install last30days@last30days-skill

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

.claude-plugin/marketplace.json

@@ -1,18 +1,23 @@
 {
   "$schema": "https://anthropic.com/claude-code/marketplace.schema.json",
-  "name": "last30days",
+  "name": "last30days-skill",
+  "description": "Research any topic from the last 30 days across Reddit, X, YouTube, TikTok, Instagram, Hacker News, Polymarket, and the web.",
   "owner": {
-    "name": "mvanhorn",
+    "name": "Matt Van Horn",
     "url": "https://github.com/mvanhorn"
   },
-  "metadata": {
-    "description": "Research any topic from the last 30 days across Reddit, X, YouTube, and the web",
-    "version": "2.1.0"
-  },
   "plugins": [
     {
       "name": "last30days",
-      "source": "./"
+      "description": "Research any topic from the last 30 days. Become an expert and write copy-paste-ready prompts.",
+      "version": "2.9.5",
+      "author": {
+        "name": "Matt Van Horn",
+        "url": "https://github.com/mvanhorn"
+      },
+      "source": "./",
+      "category": "productivity",
+      "homepage": "https://github.com/mvanhorn/last30days-skill"
     }
   ]
 }