fix: Restore correct marketplace.json source path

The source field must be a relative path starting with ./ not a GitHub URL.
GitHub marketplace installation uses shorthand format:
  /plugin marketplace add thedotmack/claude-mem

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.claude-plugin/marketplace.json

@@ -11,7 +11,7 @@
     {
       "name": "claude-mem",
       "version": "4.0.5",
-      "source": "https://github.com/thedotmack/claude-mem",
+      "source": "./plugin",
       "description": "Persistent memory system for Claude Code - context compression across sessions"
     }
   ]