feat(power): re-apply battery charge limit on AC state change

The threshold was written boot-only, so a firmware that clears
charge_control_end_threshold on unplug would silently lose the cap
until the next reboot. Add a services.udev rule on the mains adapter
(ATTR{type}=="Mains", vendor-neutral) that restarts the oneshot via
systemctl --no-block on every AC state change. Uses restart (not
try-restart) so it re-applies even if the boot run was inactive.

Eval-verified: rule present when the charge limit is on, absent when
off; downstream-template-system still evaluates.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos/options.nix

@@ -109,8 +109,9 @@
           where the hardware exposes a charge threshold
           (/sys/class/power_supply/BAT*/charge_control_end_threshold).
           null leaves charging at the firmware default. Backend-independent
-          (a small systemd unit writes the sysfs knob at boot), so it
-          works under PPD too; needs nomarchy.system.power.laptop.
+          (a small systemd unit writes the sysfs knob, re-applied on AC
+          state changes), so it works under PPD too; needs
+          nomarchy.system.power.laptop.
         '';
       };
     };