build-agent serves its seat's verbs: current, kill, pause, resume (hq ADR 0219)

The node-build-agent seat now promises these verbs, and a holder that does not
name them cannot hold it. The binary serving them is mesh-controller's
cmd/mesh-builder; merge only once a controller carrying the seat's new verbs runs,
since an older one refuses a claim naming verbs its row lacks.

modules/build-agent/module.json

@@ -8,7 +8,8 @@
   "claims": [
     {
       "name": "node-build-agent",
-      "scope": "node"
+      "scope": "node",
+      "serves": ["current", "kill", "pause", "resume"]
     }
   ],
   "requires": [