Add zlh-artifacts section — catalog-driven runtime and addon distribution

UPSTREAMS.md

@@ -21,3 +21,35 @@
 - Shadow backup system
 
 No external reverse proxy required for internal upload.
+
+---
+
+## zlh-artifacts (Artifact Server)
+
+Role:
+Central artifact distribution for runtime and addon installs.
+
+Artifacts currently hosted:
+
+```
+devcontainer/
+  _catalog.json
+  go/
+  node/
+  python/
+  java/
+  dotnet/
+
+addons/
+  code-server/
+```
+
+Used by: zlh-agent
+
+Artifacts are downloaded during provisioning using:
+
+```
+provcommon.BuildArtifactURL(...)
+```
+
+Containers do not rely on prepositioned artifacts.