battery: van-battery daemon (low-charge Pushover alerts + safe shutdown)

While on battery (AC0/online=0), sends escalating Pushover alerts at 25/20/15%
and at 10% sends a final alert then `systemctl poweroff` (after shutdown_grace
so the alert flushes first). Edge-triggered per discharge episode; re-arms when
mains returns; plug-out already-low collapses to a single alert (no burst).

Pushover token+user live only in /etc/van-battery/pushover.json (0600), seeded
from pushover.json.example on first deploy and never committed. Missing/
placeholder creds disable sending but NOT the shutdown — running flat must
always power down safely. Publishes /run/van-battery/state.json. Stdlib only.

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

power/battery-config.json

@@ -0,0 +1,9 @@
+{
+  "poll_interval": 30,
+  "ac_path": "/sys/class/power_supply/AC0/online",
+  "battery_path": "/sys/class/power_supply/BAT0",
+  "warn_levels": [25, 20, 15],
+  "shutdown_level": 10,
+  "shutdown_grace": 8,
+  "credentials_path": "/etc/van-battery/pushover.json"
+}