docs(mcp): add grepai semantic search and call graph documentation

- Add grepai section in ultimate-guide.md (Section 8.3)
  - Privacy-first semantic code search using Ollama embeddings
  - Call graph analysis: trace callers, callees, dependency graphs
  - Combined workflow with Serena for exploration + editing
  - Setup instructions and MCP tools reference
- Update decision tree and comparison table to include grepai
- Add grepai config to examples/config/mcp.json
- Document changes in CHANGELOG.md [Unreleased]

Source: https://github.com/yoanbernabeu/grepai

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

examples/config/mcp.json

@@ -28,6 +28,11 @@
         "postgresql://user:password@localhost:5432/database"
       ],
       "description": "PostgreSQL database queries"
+    },
+    "grepai": {
+      "command": "grepai",
+      "args": ["mcp-serve"],
+      "description": "Semantic code search and call graph analysis (requires Ollama)"
     }
   }
 }