feat(agent): zfs collector for pools + datasets/snapshots with fixture tests

agent/test/fixtures/zfs/zpool_status.json

@@ -0,0 +1,45 @@
+{
+  "output_version": { "command": "zpool status", "vers_major": 0, "vers_minor": 1 },
+  "pools": {
+    "rpool": {
+      "name": "rpool",
+      "state": "ONLINE",
+      "scan": {
+        "function": "scrub",
+        "state": "FINISHED",
+        "end_time": "Sat Apr 19 02:00:00 2026"
+      },
+      "error_count": "0",
+      "vdevs": {
+        "mirror-0": {
+          "name": "mirror-0",
+          "vdev_type": "mirror",
+          "state": "ONLINE",
+          "read_errors": "0",
+          "write_errors": "0",
+          "checksum_errors": "0"
+        }
+      }
+    },
+    "tank": {
+      "name": "tank",
+      "state": "DEGRADED",
+      "scan": {
+        "function": "scrub",
+        "state": "FINISHED",
+        "end_time": "Tue Mar 01 08:00:00 2026"
+      },
+      "error_count": "2",
+      "vdevs": {
+        "raidz2-0": {
+          "name": "raidz2-0",
+          "vdev_type": "raidz2",
+          "state": "DEGRADED",
+          "read_errors": "0",
+          "write_errors": "0",
+          "checksum_errors": "2"
+        }
+      }
+    }
+  }
+}