fix: add plugin.json to root .claude-plugin directory

Claude Code's plugin discovery looks for plugin.json at the
marketplace root level in .claude-plugin/, not nested inside
plugin/.claude-plugin/. Without this file at the root level,
skills and commands are not discovered.

This matches the structure of working plugins like claude-research-team.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -0,0 +1,17 @@
+{
+  "name": "claude-mem",
+  "version": "9.0.6",
+  "description": "Persistent memory system for Claude Code - seamlessly preserve context across sessions",
+  "author": {
+    "name": "Alex Newman"
+  },
+  "repository": "https://github.com/thedotmack/claude-mem",
+  "license": "AGPL-3.0",
+  "keywords": [
+    "memory",
+    "context",
+    "persistence",
+    "hooks",
+    "mcp"
+  ]
+}