Docs: the two environment models point at each other

docs/existing-environments.md

@@ -1,6 +1,12 @@
 # Taking on existing environments
 
-*Status: decided 2026-10-20; being built on the `driver-environments` branches (core and web).*
+*Status: decided 2026-10-20; built (DirXMLDev 0.49.0).*
+
+This note is about one tree deployed to every environment, with what differs kept in overrides. The
+other way, a tree per environment with changes promoted between them, is
+[promotion.md](promotion.md); DirXMLDevWeb's
+[git-workflow.md](https://github.com/PointBlueTechnology/DirXMLDevWeb/blob/main/docs/git-workflow.md#two-ways-to-keep-environments)
+compares the two.
 
 The usual case is not a clean start. A customer has a production tree and one or more others (QA,
 test, dev), each changed by hand over the years: drivers added in one and not the other, the same

docs/promotion.md

@@ -8,7 +8,9 @@ DirXMLDevWeb's [project-per-environment.md](https://github.com/PointBlueTechnolo
 
 The other model, one tree deployed to every environment with
 `overrides/<env>.properties`, works as before
-([existing-environments.md](existing-environments.md)).
+([existing-environments.md](existing-environments.md)). DirXMLDevWeb's
+[git-workflow.md](https://github.com/PointBlueTechnology/DirXMLDevWeb/blob/main/docs/git-workflow.md#two-ways-to-keep-environments)
+compares the two and says when to choose which.
 
 The examples use two fictional projects: `acme-qa/tree` (the source) and
 `acme-prod/tree` (the target).