Let Dev Updater supervise the backend, now that the switch can be sequenced

Re-applies the change reverted in d5a0f67. It was right then and only
mistimed: something was reading this working tree live, so deleting the
script took the backend card to "couldn't check" immediately rather than
on a pull. Iris has now uninstalled the service while the script was still
the declared one, which is the step that stops the old unit being orphaned
under a name nothing points at any more.

`Managed` runs the command with dev-updater's built-in service script,
resolved against the component's cwd. ai-app's own script was the generic
case exactly -- ExecStart=$BINARY with no arguments and no environment --
so it was 233 lines of init-system detection kept in step with an
identical copy next door.

Deleting it loses nothing, and that was checked rather than assumed: all
three findings the test guest produced today are in the built-in --
reading OpenRC's status exit code instead of grepping text it prints to
stderr, treating an uninitialised user softlevel as "couldn't find out"
rather than as stopped, and assigning through `|| code=$?` so `set -e`
cannot kill the script before it reads the code.

The one thing the script said that the built-in cannot is already in
AGENTS.md: 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.

Install from the backend card after taking this.

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