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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "claude-mem",
-  "version": "4.0.3",
+  "version": "4.0.4",
   "description": "Memory compression system for Claude Code - persist context across sessions",
   "keywords": [
     "claude",