Let Dev Updater supervise the backend instead of shipping a script

dev-updater now carries a built-in service implementation, generated from
a template and driven through the identical interface a project-supplied
script uses, so a project whose service is unremarkable no longer writes
one. ai-app's was unremarkable: `ExecStart=$BINARY` and `command="$BINARY"`
with no arguments and no environment. 233 lines of it, and the half that
matters most -- the OpenRC branch, which neither project can exercise from
a systemd machine -- existed twice, so a fix found by testing would have
had two places to land and no way to notice the second.

The field keeps its name; `Managed` takes the command, resolved against
the component's `cwd`.

The one thing the script said that the built-in cannot is kept, in
AGENTS.md rather than lost: Stop on this card takes down the server a
phone reaches through the tunnel, while Dev Updater itself is unaffected
because it uses its own port -- which is exactly what makes that button
easy to press and easy to regret.

NOT SAFE TO PULL BLIND. A managed service is named after the component, so
this one becomes `app-backend` while the installed one still has the name
the script gave it. Uninstall from the backend card *before* taking this
change, then Install after; pulling first orphans a service that stays
enabled and starts at boot with nothing pointing at it.

AGENTS.md

@@ -48,10 +48,21 @@ repo is in PLAN.md's "Backend layout" section.
   event model mirror; `ServerConfig.kt` settings + Keystore-sealed token;
   screens in `SessionListScreen/SessionScreen/SpawnScreen/SettingsScreen`.
 - `.dev-updater.ron` — what Dev Updater is asked to do with this checkout:
-  the backend (built in `server/`, installed and controlled through
-  `server/service`) and then the APK (built in `app/`), in that order. The
-  project it serves is the repository, not either half of it, which is why
-  this sits at the root rather than in `app/`.
+  the backend (built in `server/`, run as `service: Managed(...)`) and then
+  the APK (built in `app/`), in that order. The project it serves is the
+  repository, not either half of it, which is why this sits at the root
+  rather than in `app/`.
+  `Managed` means Dev Updater supervises `ai-server` with its own built-in
+  service implementation rather than a script kept here. ai-app had such a
+  script until 2026-08-28 and it was the generic case exactly — no
+  arguments, no environment — so the two projects were maintaining one
+  behaviour twice, including the OpenRC branch neither can test from a
+  systemd machine.
+  Worth knowing before pressing it: **Stop** on the backend card stops the
+  server that a phone reaches through the tunnel, so on that phone it stays
+  down until someone starts it again from Dev Updater. Dev Updater reaches
+  it over its own port and is unaffected, which is what makes the button
+  safe to press and easy to regret.
 - `wg-app-link/` — a **git submodule**, and the half of this backend that
   dev-updater also needed: the pinned CA and leaf (`certs`), QR enrollment
   and the bearer token (`enroll`), wg0 binding and the certificate's SANs