Let a caller say how long to wait, and note where the artifact-store test

went

`exec` waited two minutes always. A build, or anything that waits on
another machine, needs longer — and a caller that cannot say so has to
split the work to fit, which is a test shaped by its harness rather than
by what it is testing.

The scenario also places a build machine when one is given, so anything
in it can ask the mesh to build something. Nothing else here would start
one.

And the mesh-runs-its-own-artifact-store test is not here. It needs a
fourth image so the module has a registry to mirror, and that is caught
behind 04-ISSUES/012 — left as a note saying where it went and why,
rather than silently deleted, because what it asserted is worth
asserting.

src/lifecycle/operate.ts

@@ -54,12 +54,18 @@ export async function exec(
   instanceId: string,
   machine: string,
   command: string[],
+  /**
+   * How long to wait. Two minutes suits a command; a build, or anything that waits on another
+   * machine, needs longer — and a caller that cannot say so has to split the work up to fit,
+   * which is a test shaped by its harness rather than by what it is testing.
+   */
+  timeoutMs = 120_000,
 ): Promise<{ stdout: string; stderr: string }> {
   const found = (await taggedInstances()).find(
     (i) => i.instanceId === instanceId && i.machine === machine,
   );
   const name = found?.name ?? machineName(instanceId, machine);
-  return incus(["exec", name, "--", ...command], 120_000);
+  return incus(["exec", name, "--", ...command], timeoutMs);
 }
 
 /**