docs(migrate): qualify Goose zero-target behavior

migrate/README.md

@@ -152,7 +152,8 @@ unrelated modules. Services are suppressed; the existing `lifecycle/cli.Action`
 owns signals, cancellation, errors and reverse cleanup. Help and invalid arguments
 never run setup. Target versions must be nonnegative decimal int64 values, with
 no extra arguments; malformed usage returns exit 2 through `cli.Run`. Zero follows
-Goose: `down-to 0` rolls everything back, while `up-to 0` is a runtime error.
+Goose: `down-to 0` rolls everything back; `up-to 0` succeeds if nothing is pending,
+but returns a runtime error when pending work reaches Goose's version validation.
 Resolver/Goose/cleanup and status Writer failures return runtime errors (exit 1).
 Supply the native command Writer and configured Goose logger for output.