docs(ci): warn against redeploying an old controller app from the agent compose

ci/jenkins/docker-compose.yml used to define the Jenkins controller and now
defines only the agent. Redeploying an application that was set up from the
old file would replace the controller with an agent. Say so in the file, and
add an upgrade path to the README: move the controller to its own service on
the same data volume first, then deploy the agent as a new application.

ci/jenkins/README.md

@@ -74,6 +74,25 @@ Jenkins runs as a Coolify service from a Compose file:
 - **Run no builds on it:** **Manage Jenkins → Nodes → Built-In Node →
   Configure**, set **Number of executors** to `0`.
 
+#### Upgrading from the old setup
+
+`ci/jenkins/docker-compose.yml` used to define the Jenkins **controller**
+(its `jenkins` service, built from `ci/jenkins/Dockerfile`, with the
+`jenkins_home` volume). It now defines only the build agent. If your Jenkins
+was deployed from that file, **don't redeploy that application** from this
+version: it would replace the controller with an agent, and Jenkins stops
+until you rebuild it. The data volume survives, but nothing serves it.
+
+Instead:
+
+1. Create a new service for the controller as above, on the stock Java 25
+   image, mounting the **same** data volume: reference the old volume by its
+   full name and mark it `external: true`, or Jenkins starts empty.
+2. Stop the old application, start the new service, and check that your jobs
+   and credentials are there.
+3. Deploy the agent (step 3) as a **new** application, then delete the old
+   one without deleting its volume.
+
 ### 2. The agent node
 
 **Manage Jenkins → Nodes → New Node**, a permanent agent: