llmgw stop, start and restart act on the whole system

Issue #3 asked for commands to stop and start the system, and 05aa21c gave
only one machine's services. Now:

- llmgw stop sets serving.stopped in the cluster's settings, live: the
  reconciler wants every model at no replicas, freeing every GPU, and the
  gateway answers every POST under /v1/ with 503 system_stopped. The control
  planes, agents and mirror keep running, and no model's definition changes.
- llmgw start clears it; every model is placed again as it asks.
- llmgw restart has every agent restart over its stream (RestartAgent), then
  the other control planes over ssh, standbys before a leader, then this
  one; engines keep serving.
- llmgw status is the cluster's summary, saying when it is stopped.

One machine's services moved to llmgw service status|start|stop|restart.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/admin-api.openapi.json

@@ -17,6 +17,9 @@
     }
   ],
   "tags": [
+    {
+      "name": "agents"
+    },
     {
       "name": "audit"
     },
@@ -100,6 +103,38 @@
     }
   ],
   "paths": {
+    "/api/v1/agents/restart": {
+      "post": {
+        "operationId": "postAgentsRestart",
+        "summary": "Have every node's agent start again; their engines keep serving",
+        "tags": [
+          "agents"
+        ],
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/Actor"
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/AgentRestartView"
+                }
+              }
+            }
+          },
+          "401": {
+            "$ref": "#/components/responses/Unauthorized"
+          },
+          "default": {
+            "$ref": "#/components/responses/Error"
+          }
+        }
+      }
+    },
     "/api/v1/audit": {
       "get": {
         "operationId": "getAudit",
@@ -3554,6 +3589,18 @@
           }
         }
       },
+      "AgentRestartView": {
+        "type": "object",
+        "description": "AgentRestartView names the nodes whose agents were told to restart.",
+        "properties": {
+          "nodes": {
+            "type": "array",
+            "items": {
+              "type": "string"
+            }
+          }
+        }
+      },
       "CanaryPercentRequest": {
         "type": "object",
         "description": "CanaryPercentRequest changes the share of a model's requests its canary takes.",
@@ -5928,6 +5975,10 @@
             },
             "description": "Judges are the guardrails' judge models with something wrong."
           },
+          "stopped": {
+            "type": "boolean",
+            "description": "Stopped says the operators have stopped the service (llmgw stop): no model runs, and requests for one are answered 503."
+          },
           "guard_off": {
             "type": "boolean",
             "description": "GuardOff says the guardrails are turned off (guard.enabled: false): no request is checked, whatever the policies say."