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/HARDWARE-QUEUE.md

@@ -140,6 +140,12 @@ QA machine), the **T14s** (webcam case).
       now reflects the real state (the `= true` comparison fix).
       — 2026-07-04 Bernardo: PASS — theme changes committed and the
       toggle self-committed.
+- [ ] **Low-battery toasts (item 13 slice)** — on battery, drain past
+      25%: one "Battery low" toast; past 10%: a critical "Battery
+      critical" toast that stays up until dismissed (swaync); plug in,
+      drain again → it re-notifies; no repeat toasts while it just keeps
+      draining. (Crossing logic VM-verified — this checks the real
+      swaync rendering in a session.)
 
 ## AMD dev box only
 - [ ] **AMD runtime bits** — VA-API (`vainfo` → radeonsi), amd-pstate EPP