feat: stage 2 — hard-monitor agent with install.sh and systemd unit ...

- hard_monitor.py: stateless collector (psutil + requests) — identity
  (hostname/os/kernel/ips with virtual interfaces filtered), cpu/load,
  memory, disks, net counters, top processes by cpu, docker containers,
  temps into extra; --once and --dry-run modes, .env parser
- install.sh: root install into /opt/hard-monitor (venv, .env 600,
  systemd unit enable --now, idempotent)
- tested end-to-end: real agent -> panel ingest -> dashboard summary
  with net rates computed from counter deltas

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