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>

README.md

@@ -214,6 +214,7 @@ two tables below are split along exactly that line.
 | `nomarchy.waybar.enable` | `true` | Nomarchy's Waybar |
 | `nomarchy.rofi.enable` | `true` | Themed rofi launcher + `nomarchy-menu` dispatcher |
 | `nomarchy.swaync.enable` | `true` | swaync notifications, themed |
+| `nomarchy.batteryNotify.enable` | `true` | Low-battery toasts at the bar's thresholds — 25% low, 10% critical (stays up until dismissed); silent no-op on machines without a battery |
 | `nomarchy.idle.enable` | `true` | hyprlock + hypridle (idle lock 5 min, display off 10, suspend 30) |
 | `nomarchy.yazi.enable` | `true` | yazi TUI file manager, themed + curated plugins |
 | `nomarchy.osd.enable` | `true` | swayosd on-screen display for volume/brightness/mute |