chore: Release v4.0.4 - Revert to local marketplace installation

Reverted from GitHub-hosted marketplace to local marketplace file installation method. This allows us to resolve issues with better-sqlite3 native module builds before enabling GitHub marketplace distribution.

Changes:
- Simplified .claude-plugin/marketplace.json (removed metadata, version fields)
- Updated README installation instructions to use local .claude-plugin/marketplace.json
- Bumped version to 4.0.4

Installation now requires cloning the repo and using:
/plugin marketplace add .claude-plugin/marketplace.json

Will restore GitHub marketplace method once native module issues are resolved.

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

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

.claude-plugin/marketplace.json

@@ -3,19 +3,11 @@
   "owner": {
     "name": "Alex Newman"
   },
-  "metadata": {
-    "description": "Alex Newman's Claude Code plugins",
-    "version": "1.0.0"
-  },
   "plugins": [
     {
       "name": "claude-mem",
       "source": "./plugin",
-      "description": "Persistent memory system for Claude Code - seamlessly preserve context across sessions",
-      "version": "4.0.3",
-      "author": {
-        "name": "Alex Newman"
-      }
+      "description": "Persistent memory system for Claude Code"
     }
   ]
 }