Fixes#1005
Without linger, user-level systemd services stop when the SSH session
ends — even though systemctl --user status shows active (running).
Changes to systemd_install():
- Try loginctl enable-linger automatically (succeeds when the process
has the required privileges)
- If loginctl fails (no privileges), print a clear, copy-pasteable
warning with the exact command the user must run
New helper: _ensure_linger_enabled()
- Fast path: checks /var/lib/systemd/linger/<user> (no subprocess)
- Auto-enable: loginctl enable-linger <user>
- Fallback: actionable warning with sudo command + restart instructions
Tests: 4 new tests in TestEnsureLingerEnabled, 205 passed total
f10e26f731