fix(marketplace): use object form for plugin source path

The plain-string source ("./mem0-plugin") parsed as a marketplace
without errors but caused /plugin install to fail with "Plugin 'mem0'
not found in any marketplace". Claude Code and Cursor require the
object form: {"source": "directory", "path": "..."}. Codex already
uses the object form (with "source": "local") in .agents/plugins.

.claude-plugin/marketplace.json

@@ -10,7 +10,10 @@
   "plugins": [
     {
       "name": "mem0",
-      "source": "./mem0-plugin",
+      "source": {
+        "source": "directory",
+        "path": "./mem0-plugin"
+      },
       "description": "Mem0 memory layer for AI applications. Add persistent memory, personalization, and semantic search to Claude workflows.",
       "version": "0.1.1"
     }

.cursor-plugin/marketplace.json

@@ -10,7 +10,10 @@
   "plugins": [
     {
       "name": "mem0",
-      "source": "./mem0-plugin",
+      "source": {
+        "source": "directory",
+        "path": "./mem0-plugin"
+      },
       "description": "Mem0 memory layer for AI applications. Add persistent memory, personalization, and semantic search.",
       "version": "0.1.1"
     }