Add an opt-in battery indicator to the CLI and TUI status bars, shown as
the first element and colour-coded by charge (green/yellow/orange/red, or
green while charging). Off by default and a no-op on machines without a
battery.
- agent/battery.py: shared psutil-backed reader with a short TTL cache,
category bucketing, and a compact 🔋/⚡ label. Fails open to
"unavailable" everywhere.
- CLI: /battery [on|off|status] toggle persisted to display.battery,
rendered first in every status-bar width tier.
- TUI: /battery slash command, config sync, a system.battery RPC polled
while enabled, and a pinned first segment in StatusRule.
