grafana: add a CLAUDE.md-size-by-agent panel to the agents dashboard

Closes #4284. hyperhive.agent.claude_md.lines is already exported (see
hive-agent's claude_md_watch module) but had no dashboard panel — this
is the display half.

Same bargauge shape as the two sibling per-agent gauges already in this
file (container storage, active time): sum by (agent), instant query,
lastNotNull reduction, since it's a continuously-live gauge rather than
a cumulative counter. Metric name uses the dotted OTLP form per this
file's own documented convention for this store.

nix/host-modules/swarm-grafana/dashboards/agents.json

@@ -658,6 +658,65 @@
           "values": false
         }
       }
+    },
+    {
+      "id": 33,
+      "type": "bargauge",
+      "title": "CLAUDE.md size by agent",
+      "description": "hyperhive.agent.claude_md.lines — current line count of each agent's own CLAUDE.md, a continuously-live gauge (not a per-turn counter, hence lastNotNull rather than increase()). The harness itself raises a loose-ends todo past 500 lines suggesting a trim, not a hard limit — this panel is visibility, not an alarm.",
+      "datasource": {
+        "type": "prometheus",
+        "uid": "@datasourceUid@"
+      },
+      "gridPos": {
+        "h": 7,
+        "w": 12,
+        "x": 0,
+        "y": 27
+      },
+      "targets": [
+        {
+          "refId": "A",
+          "datasource": {
+            "type": "prometheus",
+            "uid": "@datasourceUid@"
+          },
+          "expr": "sum by (agent) ({__name__=\"hyperhive.agent.claude_md.lines\", hive=~\"$hive\", agent=~\"$agent\"})",
+          "instant": true,
+          "legendFormat": "{{agent}}"
+        }
+      ],
+      "fieldConfig": {
+        "defaults": {
+          "unit": "short",
+          "decimals": 0,
+          "color": {
+            "mode": "fixed",
+            "fixedColor": "orange"
+          },
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              {
+                "color": "orange",
+                "value": null
+              }
+            ]
+          }
+        },
+        "overrides": []
+      },
+      "options": {
+        "displayMode": "basic",
+        "orientation": "horizontal",
+        "showUnfilled": true,
+        "valueMode": "text",
+        "reduceOptions": {
+          "calcs": ["lastNotNull"],
+          "fields": "",
+          "values": false
+        }
+      }
     }
   ]
 }