batsaver: switch to pkexec helper, drop broken udev permission hack

The wheel-write-via-udev approach for charge_control_end_threshold has
been broken since 2026-04-08: the audit-remediation commit added
ACTION=="add" to the rule, but the threshold attribute doesn't exist
yet at the add event on Lenovo, so chmod fails silently and permissions
are never set. moonarch-batsaver-toggle has been returning Permission
denied since.

Replace the udev-rule approach with a pkexec helper:

  defaults/bin/moonarch-batsaver-apply    privileged: validate + write
  defaults/bin/moonarch-batsaver-toggle   user: read sysfs, dispatch via pkexec
  defaults/bin/moonarch-batsaver-restore  boot-time root restore (extracted
                                          from inline ExecStart for clarity)

Default Standard-pkexec prompt — password cached per session for the
~5min auth window; no polkit no-password rule, no privilege escalation
surface from misvalidated input. Same pattern Battery-Health-Charging
GNOME extension uses.

The boot-time restore service now skips the kernel write when the
sysfs value already matches the saved state (Lenovo drivers reject
same-value writes with EINVAL).

DECISIONS.md documents the failure analysis and trade-offs.
CLAUDE.md updated to describe the new flow.
moonarch-doctor: udev-effectiveness check removed.

defaults/etc/systemd/system/moonarch-batsaver.service

@@ -9,11 +9,7 @@ ConditionPathExists=/var/lib/moonarch/batsaver-threshold
 
 [Service]
 Type=oneshot
-# Validate the threshold (integer 1–100) before writing. The state file is
-# written by wheel-group users via moonarch-batsaver-toggle; the kernel rejects
-# non-numeric values on sysfs, but validating here prevents noise on boot and
-# makes the trust boundary explicit.
-ExecStart=/bin/sh -c 'V=$(cat /var/lib/moonarch/batsaver-threshold); case "$V" in ""|*[!0-9]*) exit 0;; esac; [ "$V" -ge 1 ] && [ "$V" -le 100 ] && printf %s "$V" > /sys/class/power_supply/BAT0/charge_control_end_threshold'
+ExecStart=/usr/bin/moonarch-batsaver-restore
 NoNewPrivileges=true
 ProtectHome=true
 PrivateTmp=true