replace README architecture SVG with generated image

Author: brianmeyer

README.md

@@ -9,7 +9,7 @@
 
 MollyGraph is a local graph-and-vector memory service for agents.
 
-![MollyGraph local runtime architecture](docs/assets/mollygraph-runtime.svg)
+![MollyGraph local runtime architecture](docs/assets/mollygraph-runtime.png)
 
 The default path is simple:
 - ingest text