A version prepares its state before it runs

The mesh derives the preparation from the module's own resource instead of each module hand-writing
a step beside it (novox/hq ADR 0135). A manifest says one word — `prepares` — and the mesh runs that
module's own program in its preparation mode, in the module's own context: the same image, the same
environment, the same mounts, because it is the same code. A published port and a fixed address are
taken away rather than copied, since the version being replaced still holds them.

One word for every kind of module: a Go binary receives `prepare` as its argument, a bundle receives
it through the runtime whose entry takes the same word. The control plane answers it like anything
else — its own schema stops being a special case, and its hand-written step is gone.

cmd/mesh-controller/main.go

@@ -76,7 +76,10 @@ func run() error {
 		return pinCommand(ctx, args[1:], true)
 	case "unpin":
 		return pinCommand(ctx, args[1:], false)
-	case "migrate":
+	// `prepare` is how the mesh asks any module to bring its state to the shape this version needs
+	// (novox/hq ADR 0135), and the control plane answers it the same way as everything else — its
+	// own schema is not a special case. `migrate` remains the word a person types.
+	case "prepare", "migrate":
 		return migrate(ctx)
 	case "node":
 		return nodeCommand(ctx, args[1:])
@@ -138,6 +141,7 @@ func usage() {
 	fmt.Fprint(os.Stderr, `mesh-controller — the control plane
 
   migrate                        bring each context's schema up to date
+  prepare                        the same, asked the way the mesh asks any module (ADR 0135)
   node add <name> [--adopted]    create a node record; --adopted: the machine is in use
   node list                      the nodes this mesh knows about
   node show <name>               what one machine reported it can do, and why