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/llm-fleet.json

@@ -194,7 +194,7 @@
       "id": 6,
       "type": "timeseries",
       "title": "Firing fleet alerts",
-      "description": "The LLM-fleet rules from shared/prometheus/rules/llm-fleet.yml. A firing alert is delivered to Signal: Prometheus -> Alertmanager (dankclaw quadlet) -> the host-side signal-relay user unit -> signal-cli, since 2026-09-17.",
+      "description": "The LLM-fleet rules from shared/prometheus/rules/llm-fleet.yml. EMPTY IS THE HEALTHY STATE: zero series means nothing is firing (ALERTS only carries active/pending alert series — do not read this panel as broken). A firing alert is delivered to Signal: Prometheus -> Alertmanager (dankclaw quadlet) -> the host-side signal-relay user unit -> signal-cli, since 2026-09-17.",
       "gridPos": {
         "h": 8,
         "w": 12,