build-agent: name mesh-builder by its path in busy, as its health does (review of #260)

Every merge into mesh-controller replaced the agents and the queue started
their checks again elsewhere (issue 594). Its server now declares busy as
mesh-builder busy (novox/hq ADR 0335 §5).

modules/build-agent/README.md

@@ -13,3 +13,8 @@ that repository's main (`build.artifacts[].context`); this module ships the pack
 
 Assign it to every machine with a container runtime. It replaces `builder`, the one-holder form of the
 same thing; retire that once this is assigned where it was.
+
+A send that replaces the agent's container does not throw away the build it runs: the container declares
+`busy` as `/usr/local/bin/mesh-builder busy` (novox/hq ADR 0335). The node-engine asks it before replacing it; from that
+ask the agent takes no new build, the node-engine leaves it running while it says a build is in hand, and
+replaces it once it says idle, or after 15 minutes at the latest.

modules/build-agent/module.json

@@ -78,6 +78,9 @@
         "timeout": "5s",
         "looks": 3,
         "grace": "60s"
+      },
+      "busy": {
+        "command": "/usr/local/bin/mesh-builder busy"
       }
     }
   ],