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>

scripts/hbc_mini.py

@@ -487,6 +487,12 @@ class CPUMonitorPlugin(MonitorPlugin):
         except Exception:
             pass
 
+        try:
+            with open("/proc/uptime") as fh:
+                data["uptime_seconds"] = int(float(fh.read().split()[0]))
+        except Exception:
+            pass
+
         return data
 
 
@@ -535,6 +541,20 @@ class MemoryMonitorPlugin(MonitorPlugin):
         total = mi.get("MemTotal", 0)
         avail = mi.get("MemAvailable", mi.get("MemFree", 0))
         free = mi.get("MemFree", 0)
+
+        # ZFS ARC is reclaimable but not included in MemAvailable; add it.
+        arc_kb = 0
+        try:
+            with open("/proc/spl/kstat/zfs/arcstats") as _f:
+                for _line in _f:
+                    _p = _line.split()
+                    if len(_p) >= 3 and _p[0] == "size":
+                        arc_kb = int(_p[2]) // 1024
+                        break
+        except (OSError, ValueError):
+            pass
+
+        avail = min(avail + arc_kb, total)
         used = total - avail
         data: Dict[str, Any] = {
             "memory_total": total * 1024,