fix(grafana): dashboard census fixes — every panel now receives data

Census: all 5 provisioned dashboards, 56 targets, every expression
re-evaluated against the live Prometheus/Loki with template vars expanded
(queries must run from the host: both APIs bind 127.0.0.1; from the dev box
that is connection-refused, which is NOT a dashboard defect).

- host-system 'Load 5m / core' was STRUCTURALLY EMPTY: bare '/' drops
  non-matching pairs and the right side collapses to {job} while the left
  carries {job,instance} -> 0 series forever. Fixed with / on(job); proven
  live (2 series).
- containers 'Memory vs limit (%)' was STRUCTURALLY EMPTY: every
  container_spec_memory_limit_bytes on this fleet is 0 (no cgroup limits),
  so the '> 0' filter can never match. Rewritten to share-of-host-total via
  label_replace join of the node exporter's MemTotal by host (the parens
  form is a PromQL parse error: nothing may follow group_left); 62 series
  live. Panel unit percentunit kept -> expression returns a fraction.
- llm-fleet 'Firing fleet alerts' is legitimately empty when healthy
  (ALERTS carries only firing/pending series); description now says so,
  per the Stale Signal rule that 0-series-on-ALERTS is not a defect.

logs-overview and observability: all targets already returning data.

shared/grafana/provisioning/dashboards/host-system.json

@@ -54,7 +54,7 @@
       "gridPos": { "h": 4, "w": 6, "x": 12, "y": 0 },
       "datasource": { "type": "prometheus", "uid": "prometheus" },
       "fieldConfig": { "defaults": { "unit": "short", "thresholds": { "mode": "absolute", "steps": [{ "color": "green", "value": null }, { "color": "yellow", "value": 0.7 }, { "color": "red", "value": 1 }] } }, "overrides": [] },
-      "targets": [{ "expr": "node_load5{job=~\"$node\"} / count by (job) (node_cpu_seconds_total{job=~\"$node\",mode=\"idle\"})", "legendFormat": "{{job}}", "refId": "A" }]
+      "targets": [{ "expr": "node_load5{job=~\"$node\"} / on(job) count by (job) (node_cpu_seconds_total{job=~\"$node\",mode=\"idle\"})", "legendFormat": "{{job}}", "refId": "A" }]
     },
     {
       "id": 4, "type": "stat", "title": "Open file handles",