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_*).

docs/HARDWARE.md

@@ -378,11 +378,10 @@ For contributors (and power users who will PR):
 ## 10. Doctor and hardware health (current vs target)
 
 **Today** (`nomarchy-doctor`): failed units, disk space, theme-state
-validity/git, generation age, snapper timer. **No** Wi‑Fi, GPU, fwupd,
-or fingerprint checks.
-
-**Target checks** (queued, all read-only, each failure prints one fix
-command — same doctor contract):
+validity/git, generation age, snapper; hardware section self-gates per
+machine (NetworkManager, audio sink, GPU smoke, fprintd, fwupd, charge
+limit, battery health, hibernate/zram). All read-only; each ✖ prints one
+fix command.
 
 | Check | Pass condition | Suggested fix line |
 |-------|----------------|--------------------|
@@ -391,7 +390,9 @@ command — same doctor contract):
 | GPU accel | `glxinfo`/`vainfo` smoke (if installed) | check `hardwareProfile` / drivers |
 | Fingerprint | if USB VID matched / fprintd unit | `fprintd-enroll` or enable option |
 | fwupd | daemon active; optional “updates pending” warn | `fwupdmgr get-updates` |
-| Battery threshold | if laptop + limit set, sysfs writable | power docs |
+| Battery threshold | if laptop + limit set, sysfs reports it | power docs / restart charge-limit unit |
+| Battery health | cycle_count and/or charge\|energy_full÷design % when sysfs exports them | report-only (warn if <70% of design) |
+| Hibernate / zram | disk swap + resume=; zram active | docs/MIGRATION.md → Enabling hibernation |
 
 ## 11. Option quick reference