fix(checkpoint): explicit pushes use full chunks, not the pre-push hint

The remembered chunk size fits chunks to the pre-push flush budget. An explicit push (entire checkpoint push, migration) has no budget, so a hint shrunk by a slow pre-push split it into one git push per ref. Unbounded batches now push full chunks and leave the hint alone.

Also document that the pre-push ConnectTimeout replaces a ~/.ssh/config value, and that an unreachable HTTPS remote on Linux reports a spent budget rather than an unreachable host.