docs(backlog): propose a quick toggle for the battery charge threshold

Raised by Bernardo. The charge-limit base already ships (batteryChargeLimit
option + systemd sysfs writer + udev re-trigger + control-center entry +
VM check); the gap is UX — it's a type-a-number system option needing a
rebuild. Filed under PROPOSED as a one-click conservation-mode toggle
(~80% ↔ 100%), ideally instant-effect, referencing the existing mechanism
so the base isn't re-implemented.

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

agent/BACKLOG.md

@@ -247,6 +247,23 @@ job: the `checks.*` VM suite + real toplevel/HM builds on every push
 *Agents: append here with a one-paragraph pitch (what/why/cost). Do not
 implement. Bernardo moves accepted items into a tier.*
 
+- **Battery charge-limit as a quick toggle** (Raised by Bernardo,
+  2026-07-07). The base already ships: `nomarchy.system.power.
+  batteryChargeLimit` writes `charge_control_end_threshold` via a
+  systemd service (+ a udev re-trigger on AC plug), with a "Battery
+  Limit" control-center entry and a `battery-charge-limit` VM check.
+  The gap is UX: today it's a *type-a-number* NixOS option that needs a
+  `sys-rebuild` to change. Bernardo wants a **toggle** for the charging
+  threshold — i.e. a one-click "conservation mode" (cap ~80% ↔ full
+  100%) rather than entering a value. Ideally instant-effect: the
+  service already `echo`es the threshold to sysfs, so a toggle could
+  write the live value + persist to `settings.power.batteryChargeLimit`
+  (night-light `--no-switch` pattern) instead of rebuilding, and
+  surface it prominently (menu row and/or a Waybar quick action).
+  Decide: keep the number-entry for the exact cap AND add an on/off
+  quick toggle, or replace with a fixed-cap toggle. Cost: small — mostly
+  a menu/writer + making the write instant; the sysfs mechanism exists.
+
 - **Portal/Flatpak camera picker still lists the internal IR sensor**
   (ROADMAP § Webcam follow-up). The shipped IR-hide is a *WirePlumber
   v4l2* rule, but Flatpak/portal apps consume cameras via the