feat: exclude ZFS ARC from memory_percent; add uptime_seconds to cpu_monitor

memory_monitor / hbc_mini: ZFS ARC is reclaimable but not reflected in
MemAvailable by the Linux kernel (not in SReclaimable). Read ARC size
from /proc/spl/kstat/zfs/arcstats and add it to available memory before
computing memory_percent and memory_used. No-op on systems without ZFS.

cpu_monitor: report uptime_seconds via psutil.boot_time() (full client)
and /proc/uptime (hbc_mini).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/client/plugins/cpu_monitor.py

@@ -118,6 +118,13 @@ class CPUMonitorPlugin(MonitorPlugin):
                     data["cpu_iowait"] = round(cpu_times.iowait, 1)
             except Exception as e:
                 self.logger.debug(f"Could not get CPU times: {e}")
+
+            # Uptime in seconds
+            try:
+                import time
+                data["uptime_seconds"] = int(time.time() - self.psutil.boot_time())
+            except Exception as e:
+                self.logger.debug(f"Could not get uptime: {e}")
             
             self.logger.debug(
                 f"Collected CPU metrics: {data.get('cpu_percent', 'N/A')}% usage"