Stopping the engine on Windows stops the whole engine

The installer runs the engine as <venv>\Scripts\python.exe, and since
Python 3.7.2 that file is a redirector that spawns the real interpreter
as a child. Stop() terminated the redirector and left the interpreter -
the process holding the cameras and the SQLite WAL - running with no
parent and nothing able to stop it. Seen on a Windows install: Quit from
the tray, and recognition still running.

The child is now started suspended, placed in a job object with
KILL_ON_JOB_CLOSE, and resumed. Cancel terminates the job, so the whole
tree goes; and the job dies with this process, so it goes even if the app
crashes. CREATE_NO_WINDOW while here: python.exe is a console program
and a GUI parent otherwise opens a black console on the shop counter.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KGcjxF1cNLcuwc3DAPcnfj

agent/pkg/engine/supervisor.go

@@ -194,9 +194,29 @@ func (s *Supervisor) runOnce(ctx context.Context) error {
 		return err
 	}
 	cmd.Stderr = cmd.Stdout
+	// Cancel ends the whole process tree, not just the process exec spawned.
+	// `kill` is filled in after Start, once the tree is confined; until then
+	// it is exec's own behaviour.
+	var kill func() error
+	cmd.Cancel = func() error {
+		if kill == nil {
+			return cmd.Process.Kill()
+		}
+		return kill()
+	}
+	prepare(cmd)
 	if err := cmd.Start(); err != nil {
 		return fmt.Errorf("engine failed to start: %w", err)
 	}
+	k, release, err := confine(cmd)
+	if err != nil {
+		// Not fatal: the engine runs, and stopping it falls back to killing
+		// the one process. Logged because on Windows that fallback is the
+		// bug this exists to fix.
+		fmt.Fprintf(s.opts.LogWriter, "supervisor: could not confine engine process tree: %v\n", err)
+	}
+	kill = k
+	defer release()
 
 	pumped := make(chan struct{})
 	go func() {