Let a build agent be paused, have a build killed, and end an ask cancelled as it took it (hq ADR 0219)

A queued ask could only be waited out and a running build only ended by
stopping the machine, which redelivered it elsewhere. The holder now serves
current, kill, pause and resume on its own machine's subjects; a kill ends
the build's process group and labelled containers and settles the ask as
failed, killed by hand; pause is kept in the workspace across a restart and
said on the bus. The controller writes cancelled ids to a cancelled set the
holder reads on taking an ask, closing the race a delete alone leaves.

internal/catalogue/seats.go

@@ -117,8 +117,16 @@ var defaultSeats = append([]Seat{
 	// **Node-scoped, and every holder takes from one queue** (novox/hq ADR 0190): a build is asked of
 	// the role, and whichever machine holding the seat is idle pulls it. One holder per machine is
 	// what the scope says; sharing the work is what a seat's queue has always done.
+	//
+	// **And its holder answers for the build it is running** (novox/hq ADR 0219): what it is
+	// building, kill it, take nothing new, take again. The queue as a whole is the controller's to
+	// show and change (`queue`, `cancel`, `clear`); what one machine does with an ask it already
+	// took only that machine can do. `paused.<node>` is each holder saying whether it takes work, so
+	// the controller can tell a plan waiting on a paused seat from one that is late.
 	{Name: "node-build-agent", Scope: ScopeNode,
-		Accepts: []string{"build"}, Emits: []string{"started", "built", "log.*"}, Decision: "novox/hq ADR 0190"},
+		Accepts: []string{"build"}, Emits: []string{"started", "built", "log.*", "paused.*"},
+		Serves:   buildAgentVerbs(),
+		Decision: "novox/hq ADR 0190, ADR 0219"},
 	// **Retired by ADR 0190, kept while a manifest still claims it.** The one build machine's seat.
 	// A claim to a seat the mesh no longer defines is refused, and the module holding this one is
 	// assigned on a live machine until build-agent replaces it — removing the row first would make
@@ -581,3 +589,23 @@ func loginShellVerbs() []Verb {
 			}, []string{"command"})},
 	}
 }
+
+// buildAgentVerbs are what a holder of node-build-agent answers on its own machine (novox/hq ADR
+// 0219). One build at a time per holder (ADR 0190), so "the build running here" is one or none.
+func buildAgentVerbs() []Verb {
+	return []Verb{
+		{Name: "current", Description: "The build this machine is running — its id, repository, path, " +
+			"the step it is at, when it started and for how long — or none; and whether this machine is " +
+			"paused, taking no new build.",
+			Input: schema(map[string]string{}, nil)},
+		{Name: "kill", Description: "End the build with this id, running here: its commands and the " +
+			"containers it started are stopped, and its outcome is announced as failed, killed by hand — " +
+			"settled, so it is not handed to another machine.",
+			Input: schema(map[string]string{"id": "the build's id, as `queue` or `current` says it"}, []string{"id"})},
+		{Name: "pause", Description: "Take no new build on this machine until resumed; a build running " +
+			"here finishes. Kept across a restart of the holder.",
+			Input: schema(map[string]string{}, nil)},
+		{Name: "resume", Description: "Take builds again on this machine.",
+			Input: schema(map[string]string{}, nil)},
+	}
+}