Fix System Info table merge and boot time unit in host-resources dashboard

Replace merge transformation with joinByField on instance — the merge
couldn't combine rows from queries with different label cardinality
(node_uname_info has many extra labels), producing 7 rows instead of 3.

Fix boot time showing "56 years ago" by multiplying node_boot_time_seconds
by 1000 — Grafana's dateTimeFromNow unit expects epoch milliseconds.

Also hide the exported_nodename column (container hostname, not useful).

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

incusos/observability-dashboards/core/host-resources.json

@@ -108,7 +108,7 @@
           "instant": true
         },
         {
-          "expr": "node_boot_time_seconds{job=\"node-exporter\", instance=~\"$node\"}",
+          "expr": "node_boot_time_seconds{job=\"node-exporter\", instance=~\"$node\"} * 1000",
           "legendFormat": "",
           "refId": "B",
           "format": "table",
@@ -234,17 +234,28 @@
       },
       "transformations": [
         {
-          "id": "merge",
-          "options": {}
+          "id": "joinByField",
+          "options": {
+            "byField": "instance",
+            "mode": "outer"
+          }
         },
         {
           "id": "organize",
           "options": {
             "excludeByName": {
-              "Time": true,
-              "__name__": true,
-              "job": true,
+              "Time 1": true,
+              "Time 2": true,
+              "Time 3": true,
+              "Time 4": true,
+              "__name__ 1": true,
+              "__name__ 2": true,
+              "job 1": true,
+              "job 2": true,
+              "job 3": true,
+              "job 4": true,
               "domainname": true,
+              "exported_nodename": true,
               "machine": true,
               "nodename": true,
               "sysname": true,