ci: run the agent with an init process; document the controller upgrade

- init: true on the agent so processes left behind by build tools are reaped,
  as the inbound-agent image asks for.
- Redeploying an older controller from docker-compose.yml upgrades Jenkins
  and moves it from Java 17 to 25: say to back up jenkins_home first, update
  plugins that fail to load, and run the job once before relying on it.

ci/jenkins/docker-compose.agent.yml

@@ -10,6 +10,9 @@ services:
       context: .
       dockerfile: Dockerfile
     restart: unless-stopped
+    # Reap the processes build tools leave behind (dotnet build servers,
+    # Gradle daemons, adb), as the inbound-agent image's docs ask for.
+    init: true
     environment:
       # Jenkins as seen from inside Docker (e.g. http://<jenkins container
       # name>:8080), not the public URL, which is behind Cloudflare Access.