systemd-run --scope does not give the invoked process a new session: the
worker keeps the parent's session and inherits its controlling terminal.
When the parent is an interactive TUI on a pts (INVOCATION_ID present ->
is_gateway_supervisor_process()=True), every background spawn drops the
worker into the same session as the foreground process group; the spawn
then stops the whole session (SIGTTIN/SIGTTOU family), observed as 5 dead
TUIs in state T ("Arrêté") on 2026-08-08.
Fix: popen_start_new_session = True in the systemd-scope branch of
spawn_local. The worker (and the systemd-run wrapper) get a private
session while the scope cgroup isolation is preserved - the scope is
attached to the invoked process, not to the spawning session.
Verified: simulated TUI (INVOCATION_ID) + ProcessRegistry.spawn_local ->
worker in hermes-worker-*.scope with sid != simulator sid, exits cleanly,
simulator stays alive (previously: same sid -> stopped).
