feat(battery): low-battery notifications at the bar's 25/10% thresholds (item 13 slice)

The bar colored the battery at 25/10% but nothing notified. New
shellcheck-gated watcher (pkgs/nomarchy-battery-notify, overlay) polls
the same sysfs the bar reads — type=Battery, scope!=Device — and fires
one toast per downward crossing (normal at 25%, critical at 10%; swaync
keeps critical up until dismissed), re-armed by charging. Self-gates on
battery presence (the powerprofile pattern), so desktops get a silent
no-op. HM user unit behind nomarchy.batteryNotify.enable (default on)
with libnotify on the unit PATH; notify-send resolves from PATH so the
VM check can shim it. poweralertd rejected: its UPower thresholds
(20/5) wouldn't match the bar, and it toasts every plug/unplug.

Verified: V0 (flake check, bash -n); V1 (template-home builds, unit in
the generation); V2 GREEN (new checks.battery-notify, test_power fake
battery: self-gate, silence at 80%, one toast per crossing, critical
urgency, charging re-arms). Remains: V3 — real swaync toast on a
draining laptop (HARDWARE-QUEUE).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkgs/nomarchy-battery-notify/default.nix

@@ -0,0 +1,12 @@
+# Low-battery notification watcher (nomarchy.batteryNotify). A package
+# (not an inline script) so the VM check can exercise the crossing logic
+# on a minimal node against a test_power fake battery — same reasoning
+# as nomarchy-doctor. libnotify is deliberately NOT a runtimeInput: the
+# user unit puts it on PATH; the check shims notify-send instead.
+{ writeShellApplication, coreutils }:
+
+writeShellApplication {
+  name = "nomarchy-battery-notify";
+  runtimeInputs = [ coreutils ];
+  text = builtins.readFile ./nomarchy-battery-notify.sh;
+}

pkgs/nomarchy-battery-notify/nomarchy-battery-notify.sh

@@ -0,0 +1,58 @@
+# Low-battery notifications — the session-side complement to the bar's
+# battery colors: Waybar paints the module @warn at 25% and @bad at 10%
+# (waybar.nix battery.states) but nothing *notified*. This watcher polls
+# the same sysfs state the bar reads and fires exactly one notification
+# per downward crossing — normal at 25%, critical at 10% (swaync keeps
+# critical toasts up until dismissed) — re-arming once the charger lands.
+#
+# notify-send is resolved from PATH on purpose (not a runtimeInput): the
+# unit (battery-notify.nix) provides libnotify; the VM check shims it to
+# capture the calls.
+#
+# usage: nomarchy-battery-notify [poll-interval-seconds]
+
+interval="${1:-30}"
+warn=25
+crit=10
+
+# System batteries only: type Battery, and not scope=Device (how
+# peripheral batteries — mice, headsets — report). Name-agnostic on
+# purpose: BAT0, dual-battery BAT0+BAT1, CMB0, test_battery all match.
+batteries() {
+  local d
+  for d in /sys/class/power_supply/*/; do
+    [ "$(cat "$d/type" 2>/dev/null)" = Battery ] || continue
+    [ "$(cat "$d/scope" 2>/dev/null || echo System)" = Device ] && continue
+    printf '%s\n' "$d"
+  done
+}
+
+# Self-gate: no battery, not a laptop — a clean no-op on desktops.
+[ -n "$(batteries)" ] || exit 0
+
+state=ok # ok → warn → crit, one notification per downward crossing
+while :; do
+  sum=0 n=0 discharging=
+  while IFS= read -r d; do
+    cap=$(cat "$d/capacity" 2>/dev/null) || continue
+    sum=$((sum + cap)); n=$((n + 1))
+    [ "$(cat "$d/status" 2>/dev/null)" = Discharging ] && discharging=1
+  done < <(batteries)
+
+  if [ "$n" -gt 0 ] && [ -n "$discharging" ]; then
+    cap=$((sum / n))
+    if [ "$cap" -le "$crit" ] && [ "$state" != crit ]; then
+      notify-send -u critical -a Nomarchy "Battery critical: ${cap}%" \
+        "Plug in now."
+      state=crit
+    elif [ "$cap" -le "$warn" ] && [ "$state" = ok ]; then
+      notify-send -u normal -a Nomarchy "Battery low: ${cap}%" \
+        "Consider plugging in."
+      state=warn
+    fi
+  else
+    # On the charger (or nothing readable): re-arm for the next drain.
+    state=ok
+  fi
+  sleep "$interval"
+done