Release v6.4.0: Dual-tag system, search API improvements, and privacy features

This release introduces powerful new privacy controls and search improvements:

**New Features:**
- Dual-tag system for meta-observation control
  - <private> tags for user-level privacy control
  - <claude-mem-context> tags for system-level auto-injected observations
- Privacy tag documentation and inline help in context hook

**Improvements:**
- Simplified search API parameters to eliminate bracket encoding issues
- Improved user messaging for new privacy features

**Technical:**
- Tag stripping happens at hook layer (edge processing)
- Shared utilities in src/utils/tag-stripping.ts
- Database index optimization for user prompts lookup

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

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

src/hooks/user-message-hook.ts

@@ -52,7 +52,8 @@ try {
     "\n\n📝 Claude-Mem Context Loaded\n" +
     "   ℹ️  Note: This appears as stderr but is informational only\n\n" +
     output +
-    "\n\n💬 Community\nhttps://discord.gg/J4wttp9vDu\n" +
+    "\n\n💡 New! Wrap all or part of any message with <private> ... </private> to prevent storing sensitive information in your observation history.\n" +
+    "\n💬 Community https://discord.gg/J4wttp9vDu\n" +
     `\n📺 Watch live in browser http://localhost:${port}/\n`
   );