feat(doctor): battery health readout (#80)

Report-only cycle_count + retained capacity (charge_* or energy_*) as
% of design on system batteries; soft warn below 70% of design; skip
when attrs missing. NOMARCHY_POWER_SUPPLY_ROOT for the checks.doctor
fixture (test_power has no cycle/design attrs).

Verified: V2 — nix flake check --no-build; checks.doctor green (fake
BAT0 cycles/% + Device-scope ignore); local BAT0 smoke (energy_*).

agent/BACKLOG.md

@@ -55,11 +55,6 @@ runtime-computed `resume_offset` into the boot cmdline, and the dev box runs
 exactly this layout (`/proc/cmdline` verified) but isn't Nomarchy. Steps in
 HARDWARE-QUEUE. Close #76 on PASS.
 
-### 80. Battery health readout (`VISION § C`)
-Report-only: `cycle_count` + `charge_full`/`charge_full_design` % in a
-doctor battery-health line (and optionally the Power menu). sysfs-gated
-so desktops/VMs skip it. Extend checks.doctor with a faked sysfs battery.
-
 ### 81. First-boot welcome card (`VISION § B`)
 One dismissible "you're set" toast on the first session (SUPER+M menu,
 SUPER+T themes, SUPER+? keys, network pointer) — a notification, **not**
@@ -145,9 +140,9 @@ high-ROI, etc.) live in the journal + ROADMAP — not here.*
   2026-07-10). Keep #59 commented install guidance; no
   `nomarchy.hardware.nvidia.*` until a hybrid maintainer + queue.
 
-_(First-boot card, generation readability, battery health, pre-activate
-  flag, dock life, hall of fame → promoted to **NEXT #80–#85**,
-  Bernardo 2026-07-11.)_
+_(#80 battery health shipped 2026-07-11. Remaining promoted: first-boot
+  card, generation readability, pre-activate flag, dock life, hall of
+  fame → **NEXT #81–#85**.)_
 
 ### v1.0 pointer