phonebox: arm hardware watchdog and auto-reboot on panic

mirach and almach have each hung at least twice since July with nothing in
the journal or pstore, and stayed down until someone power-cycled the board.
RuntimeWatchdogSec was 0 and kernel.panic was 0, so a hung kernel/PID1 or a
panic could never self-recover.

Arm the systemd hardware watchdog (30s) and reboot 10s after a panic on all
phonebox nodes. Watchdog resets will show up as extra entries in
journalctl --list-boots.

modules/clan/phonebox/default.nix

@@ -256,6 +256,13 @@
               '';
             };
 
+            # Phonebox nodes are headless and unattended. Observed on mirach/almach
+            # (2026-07..09): hard hangs leave no journal/pstore trace and stay down
+            # until someone power-cycles the board. Arm the hardware watchdog so a
+            # hung kernel/PID1 resets itself, and reboot instead of halting on panic.
+            systemd.settings.Manager.RuntimeWatchdogSec = "30s";
+            boot.kernel.sysctl."kernel.panic" = 10;
+
             networking.interfaces = {
               ${ata-interface} = {
                 useDHCP = false;