swarm-grafana: add distinct-subagent-count stat panel

mara: "show subagent count in time range" — the existing 'Subagent turns'
tile answers volume (how many turns), not this (how many differently-named
subagents ran at all). New panel-45 counts subagent label values with at
least one turn in the selected range.

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

@@ -1338,6 +1338,72 @@
             }
           }
         }
+      },
+      "panel-45": {
+        "kind": "Panel",
+        "spec": {
+          "id": 45,
+          "title": "Distinct subagents (range)",
+          "description": "How many differently-named subagents ran at all in the selected range, not how many turns they took — 'Subagent turns' above already covers volume, this answers a different question (mara: \"show subagent count in time range\"). Counts subagent label values with at least one turn (increase(...) > 0) rather than every label value claude_code.session.count has ever carried, so a stale series with no turns in-range doesn't inflate the count.",
+          "links": [],
+          "data": {
+            "kind": "QueryGroup",
+            "spec": {
+              "queries": [
+                {
+                  "kind": "PanelQuery",
+                  "spec": {
+                    "query": {
+                      "kind": "DataQuery",
+                      "group": "prometheus",
+                      "version": "v0",
+                      "datasource": {
+                        "name": "@datasourceUid@"
+                      },
+                      "spec": {
+                        "expr": "count(count by (subagent) (increase({__name__=\"claude_code.session.count\", hive=~\"$hive\", agent=~\"$agent\", subagent!=\"\"}[$__range]) > 0))",
+                        "instant": true,
+                        "legendFormat": "distinct subagents"
+                      }
+                    },
+                    "refId": "A",
+                    "hidden": false
+                  }
+                }
+              ],
+              "transformations": [],
+              "queryOptions": {}
+            }
+          },
+          "vizConfig": {
+            "kind": "VizConfig",
+            "group": "stat",
+            "version": "",
+            "spec": {
+              "options": {
+                "colorMode": "value",
+                "graphMode": "none",
+                "reduceOptions": {
+                  "calcs": ["lastNotNull"],
+                  "fields": "",
+                  "values": false
+                },
+                "textMode": "value"
+              },
+              "fieldConfig": {
+                "defaults": {
+                  "unit": "short",
+                  "decimals": 0,
+                  "color": {
+                    "mode": "fixed",
+                    "fixedColor": "text"
+                  }
+                },
+                "overrides": []
+              }
+            }
+          }
+        }
       }
     },
     "layout": {
@@ -1611,6 +1677,19 @@
                           "name": "panel-44"
                         }
                       }
+                    },
+                    {
+                      "kind": "GridLayoutItem",
+                      "spec": {
+                        "x": 0,
+                        "y": 11,
+                        "width": 8,
+                        "height": 4,
+                        "element": {
+                          "kind": "ElementReference",
+                          "name": "panel-45"
+                        }
+                      }
                     }
                   ]
                 }