A build says what it does on the bus, as it happens (novox/hq ADR 0157)

The build-machine seat emits `started` and `log.<build id>` beside `built`. Every line the builder
speaks — each step, each command with its duration, and on failure the command's own output — goes
to stderr as before and onto the bus under the build's id, one subject per build, kept a week in
EVENTS with every other event. `builds --log <id>` reads it back from the stream with a consumer
that is gone when the reading is done, on the command line and as the controller's seat verb;
`builds` lists each build's id and `build` says the id it asked with.

Lines are core publishes with a sequence number, so a build is not slowed by an ack per line and a
gap is visible; `started` and `built` are awaited into the stream. The seat protocol widens
additively at the controller's next start; the holder's grant follows on the broker node's next
composition.

internal/builder/builder.go

@@ -719,6 +719,21 @@ func short(commit string) string {
 	return commit
 }
 
+// Said is where the lines Command speaks go, beside the build's own Log: what runs, how long it
+// took, and that it failed. Nil prints them to stderr, as a build machine with nobody listening
+// should. The machine sets it per build so every line reaches the bus too (novox/hq ADR 0157) —
+// the step is "run", and the message is the line as it has always been printed.
+var Said Log
+
+func tell(step, format string, args ...any) {
+	message := fmt.Sprintf(format, args...)
+	if Said == nil {
+		fmt.Fprintf(os.Stderr, "  %s\n", message)
+		return
+	}
+	Said(step, message)
+}
+
 // Command is a Runner that actually runs things.
 func Command(ctx context.Context, dir, name string, args ...string) (string, error) {
 	// **Every command is echoed before it runs**, with where. On a build that hangs, the last line
@@ -726,16 +741,23 @@ func Command(ctx context.Context, dir, name string, args ...string) (string, err
 	// nothing" and "git clone is waiting on a network that will not answer". Silent on success is
 	// what made an empty workspace unreadable.
 	started := timeNow()
-	fmt.Fprintf(os.Stderr, "  $ (%s) %s %s\n", short(filepath.Base(dir)), name, strings.Join(args, " "))
+	tell("run", "$ (%s) %s %s", short(filepath.Base(dir)), name, strings.Join(args, " "))
 	cmd := exec.CommandContext(ctx, name, args...)
 	cmd.Dir = dir
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "  ! %s %s failed after %s\n", name, args[0], since(started))
+		tell("run", "! %s %s failed after %s", name, args[0], since(started))
+		// The command's own output is part of what a reader needs — the compiler's error, the
+		// clone's refusal — and a line per output line keeps it readable on the bus.
+		for _, line := range strings.Split(strings.TrimSpace(string(out)), "\n") {
+			if line != "" {
+				tell("output", "%s", line)
+			}
+		}
 		return string(out), fmt.Errorf("%s %s: %w\n%s",
 			name, strings.Join(args, " "), err, strings.TrimSpace(string(out)))
 	}
-	fmt.Fprintf(os.Stderr, "  ✓ %s %s (%s)\n", name, firstArg(args), since(started))
+	tell("run", "✓ %s %s (%s)", name, firstArg(args), since(started))
 	return string(out), nil
 }