fix: correct ZFS pool status threshold operator and add per-metric grace

The default zfs_monitor.*.status threshold used operator '>' with warning=1,
so a DEGRADED pool (status=1) never alerted (1 > 1 is false) and a FAULTED
pool (status=2) only triggered WARNING instead of CRITICAL.

Fix the operator to '>=' in THRESHOLD_DEFAULTS and the example config.

Also adds a per-metric grace period override (ThresholdConfig.grace) so
individual thresholds can bypass or shorten the global grace delay. Alerts
with grace=0 fire immediately on state change rather than waiting for a
second collection cycle. Sets grace=0 on zfs_monitor.*.status so pool
degradation alerts fire on the first data report after the event.

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

hbd/config_thresholds_example.yaml

@@ -146,8 +146,9 @@ thresholds:
         status:
           warning: 1           # Alert WARNING when pool is DEGRADED
           critical: 2           # Alert CRITICAL when pool is SUSPENDED/FAULTED/UNAVAIL
-          operator: ">"
-          hysteresis: 0.0       # No hysteresis — a degraded pool is always critical
+          operator: ">="
+          hysteresis: 0.0       # No hysteresis — a degraded pool is always alerting
+          grace: 0              # Fire immediately — don't wait for a second collection
           display: "ZFS pool {pool_name} is {health}"
 
       # Per-pool capacity thresholds (optional; add pools you care about)