feat(idle): suspend on battery only, cohering with power management

hypridle's suspend listener now runs `nomarchy-on-ac || systemctl
suspend`, so a plugged-in machine no longer suspends itself mid-idle
(long builds, media, presentations survive). It also fires sooner —
15 min, vs the old fixed 30 — since it now only applies unplugged.
Lock and screen-off are unchanged on both power sources, and logind's
lid handling stays at its sensible defaults (suspend on lid close,
ignore when docked).

Closes the idle-cohesion follow-up under the laptop-power roadmap item.

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

modules/home/idle.nix

@@ -2,13 +2,23 @@
 # theme-state.json. One concern, one file: hypridle drives WHEN (idle
 # lock, display off, suspend, lock-before-sleep), hyprlock is the
 # themed lock screen itself (also behind the power menu's Lock entry).
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 let
   cfg = config.nomarchy;
   t = cfg.theme;
   c = t.colors;
   inherit (config.nomarchy.lib) rgb;
+
+  # Exits 0 when running on AC: a mains adapter reports `online` 1 (battery
+  # supplies have no `online` node, so they never match). Used as
+  # `${onAc} || <action>` so the action is skipped while plugged in.
+  onAc = pkgs.writeShellScript "nomarchy-on-ac" ''
+    for f in /sys/class/power_supply/*/online; do
+      [ -r "$f" ] && [ "$(cat "$f")" = "1" ] && exit 0
+    done
+    exit 1
+  '';
 in
 {
   config = lib.mkIf cfg.idle.enable {
@@ -62,13 +72,20 @@ in
           after_sleep_cmd = "hyprctl dispatch dpms on";
         };
         listener = [
+          # Lock and screen-off are the same on either power source —
+          # they're about privacy and the panel, not battery.
           { timeout = 300; on-timeout = "loginctl lock-session"; }
           {
             timeout = 600;
             on-timeout = "hyprctl dispatch dpms off";
             on-resume = "hyprctl dispatch dpms on";
           }
-          { timeout = 1800; on-timeout = "systemctl suspend"; }
+          # Suspend only on battery, and sooner than the old fixed 30 min
+          # (it now only fires unplugged). Plugged in, the machine stays
+          # up — long builds, media, presentations aren't killed mid-idle.
+          # Closing the lid still suspends on AC (logind's default): that's
+          # an explicit "I'm done", distinct from sitting idle.
+          { timeout = 900; on-timeout = "${onAc} || systemctl suspend"; }
         ];
       };
     };