The builder narrates every step, and every command it runs

A build was silent from clone to publish, so a build in progress, one that failed
quietly, and a request that never arrived all looked identical — which cost a long
diagnosis against a running mesh chasing "the handler never fired".

Now: the handler announces a request the instant it lands. Build logs each phase
— clone, commit, manifest, bases, each artifact starting and finishing with what
it produced, resolve, done — through a Log callback that is nil-safe, so the tests
that pass none still build. And the Command runner echoes every command before it
runs, with where and how long it took, because on a hang the last line is exactly
the command it is stuck inside: "git clone waiting on a network that will not
answer" rather than "the builder did nothing".

The unreadable-request path prints to stdout now too, not stderr, so it shows in
docker logs without splitting streams — the split is what hid it.

Claude-Session: https://claude.ai/code/session_01D6qtiYU3P9jk3pnAXyAFyx

cmd/mesh-builder/once.go

@@ -84,7 +84,8 @@ func buildOnce(ctx context.Context, args []string) error {
 	}
 	fmt.Fprintln(os.Stderr)
 
-	built, buildErr := builder.Build(ctx, builder.Command, publisher, repository, *path, *ref, where, bases)
+	built, buildErr := builder.Build(ctx, builder.Command, publisher, repository, *path, *ref, where, bases,
+		func(step, message string) { fmt.Printf("  [%s] %s\n", step, message) })
 	if buildErr != nil {
 		return buildErr
 	}