docs(fern): fix architecture diagram image (#792)

Use a standard Markdown image so the architecture diagram renders correctly in the published Fern docs.

Author: pimlock

fern/pages/about/architecture.mdx

@@ -15,8 +15,7 @@ position: 2
 
 OpenShell runs inside a Docker container. Each sandbox is an isolated environment managed through the gateway. Four components work together to keep agents secure.
 
-```{figure} architecture.svg
-```
+![OpenShell architecture diagram](/assets/images/architecture.svg)
 
 ## Components