Add watch mode to `mm-snap sandbox` command

For a better developer experience, we should add a watch mode (e.g., `mm-snap sandbox --watch`) to the sandbox command, maybe enabled by default. This should only work if `--no-build` (or `--build false`) is not set.

Start at the `mm-snap sandbox` command entry point and inspect how its build and `--no-build`/`--build false` options are handled. Determine the intended watch behavior and whether it should be enabled by default; done means sandbox changes are watched when building is enabled and watch mode remains unavailable when it is disabled.

Written by the indexing model from the issue text.