Handle the case where GC is disabled but a repo limit is configured

When a repo size limit is configured but gc is disabled (i.e., no `--enable-gc` flag has been passed), we should either (or both?):

1. Warn on start.
2. Refuse to write to the datastore once the size has been exceeded.

Start by tracing the daemon startup handling for the repo size limit and the --enable-gc flag, then follow the datastore write path. Determine whether the intended behavior is a startup warning, refusing writes after the limit is exceeded, or both, and add coverage for the selected behavior.

Written by the indexing model from the issue text.