Add Claude Code marketplace metadata for paper-reader skill

Moves skill/ to skills/paper-reader/ and adds .claude-plugin/
marketplace and plugin manifests so the repo can be installed
directly as a Claude Code skill marketplace.

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

.claude-plugin/marketplace.json

@@ -0,0 +1,14 @@
+{
+  "name": "paper-marketplace",
+  "owner": {
+    "name": "Ellie"
+  },
+  "plugins": [
+    {
+      "name": "paper",
+      "source": "./",
+      "description": "Fetch and read academic papers by DOI, converting to markdown",
+      "version": "0.1.0"
+    }
+  ]
+}