Clamp Linux battery capacity percentage

[VACInc]

Summary

• clamp Linux sysfs battery capacity values to the valid 0-100 range before reporting them
• add a regression test for a kernel/sysfs capacity value above 100

Context

Some Linux systems can briefly report battery capacity values above 100 via /sys/class/power_supply/*/capacity. Beszel currently forwards that value directly, which can surface impossible percentages like 105% in integrations.

Tests

• go test -tags testing ./agent/battery
• go test ./agent/...