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>

agent/BACKLOG.md

@@ -28,11 +28,8 @@ next, in what order*.
 
 ### 13. Small niceties batch (one slice per iteration)
 Each is a small, self-contained polish item in the existing patterns
-(idle-inhibit/caffeine slice shipped 2026-07-04 — generated bar +
-summer-day parity + cheatsheet row):
-- **Low-battery notifications:** the bar colors at 25/10% but nothing
-  *notifies*; gate on `power.laptop` (poweralertd, or a small upower
-  watcher consistent with how the bar reads state).
+(shipped: idle-inhibit/caffeine 2026-07-04; low-battery notifications
+2026-07-05 — sysfs watcher + checks.battery-notify):
 - **Color picker:** `hyprpicker` → clipboard as a Tools › entry +
   `SUPER+CTRL` bind; pairs naturally with theme work.