feat: health check suite — CPU, memory, interface errors, log alerts

Composable health checks that work across vendors.

- CPU utilization check with threshold alerting
- Memory utilization check
- Interface error counter check (CRC, input/output errors, drops)
- Log scanning for critical/major events
- JSON output for monitoring integration

```bash
python -m netops.check.health --inventory inv.yaml --group core --threshold cpu=80,mem=85
```