docs: add doobidoo/mcp-memory-service documentation (semantic memory)

- Add comprehensive section in ultimate-guide.md (L.6385-6523)
- Document 3 backends: sqlite_vec, cloudflare, hybrid
- List 12 MCP tools with descriptions
- Add environment variables reference
- Include multi-client sync patterns (same machine vs multi-device)
- Add disclaimer: under testing, not yet validated in production
- Update reference.yaml with 8 new entries
- Add to cheatsheet.md and mcp.json template

Complements Serena (key-value) with semantic search capabilities.

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

examples/config/mcp.json

@@ -33,6 +33,11 @@
       "command": "grepai",
       "args": ["mcp-serve"],
       "description": "Semantic code search and call graph analysis (requires Ollama)"
+    },
+    "doobidoo-memory": {
+      "command": "memory",
+      "args": ["server"],
+      "description": "Semantic memory with cross-session search (complements Serena)"
     }
   }
 }