Skip reboot prompts in unattended updates

[maxcroy1]

Summary

• skip gum confirm when an update is unattended or has no usable input/output terminal
• keep reporting why a reboot is needed and preserve the interactive reboot offer
• exercise the real restart helper with fail-closed stubs for interactive, unattended pseudo-TTY, and headless calls

Evidence

Before the fix, the isolated regression test passed its interactive control and then proved that -y still reached the prompt inside the pseudo-TTY created by script(1):

ok - interactive restart check offers the reboot on the update pseudo-terminal
gum stdin=yes stdout=yes unattended=1 args=confirm Linux kernel has been updated. Reboot?
shell restart
not ok - -y reaches the post-update reboot prompt

The failure reproduced four out of four times. The harness used a fake HOME, forced a harmless kernel mismatch, and stubbed uname, pacman, gum, pgrep, omarchy-system-reboot, omarchy-state, and restart commands. No real prompt, package operation, state change, restart, or reboot was attempted.

After the fix:

ok - interactive restart check offers the reboot on the update pseudo-terminal
ok - -y reports the required reboot without asking
ok - headless restart check reports the required reboot without asking

Testing

• bash test/shell.d/update-restart-test.sh in a Bubblewrap sandbox: passes
• ./test/cli in the same sandbox: passes
• ./test/all in the sandbox: the new test and existing update sequence coverage pass; the aggregate remains nonzero on five unrelated environment checks that require the separate omarchy-pkgs checkout, a live network route, or absence of an installed URL-check helper

The sandbox used a read-only checkout, private user/PID/network/IPC namespaces, temporary home/runtime filesystems, minimal devices, cleared desktop/session variables, and no access to the live Wayland, Hyprland, D-Bus, or user-service session.

Fixes #8986