fix(checkpoint): push git-refs checkpoints in bounded chunks

The pre-push hook pushed every queued checkpoint ref in one git push. On Windows that command line passes the 32,767-character CreateProcess limit at ~340 refs (each refspec is ~95 characters), so the batch failed before git started and every push fell back to per-ref retries that drain only a few dozen refs per 2-minute budget (#2986). Elsewhere a large backlog or a stalled connection could hold the user's push with no bound.

• Push queued refs at most 200 per git push (~19k characters), each chunk's queue entries removed as soon as it lands.
• Share one checkpointFlushBudget across the batch and the per-ref fallback in pre-push; the explicit pushes (migration, entire checkpoint push) leave the batch unbounded.
• A failed chunk no longer stops later chunks; only its refs are retried individually. Batching stops after 2 consecutive failed chunks, an SSH auth failure, the budget, or an interrupt.
• Stop after one attempt when git cannot connect at all (connect-phase ssh/curl failures before anything landed), printing that line with credentials redacted.
• Add -o ConnectTimeout=60 for OpenSSH under the pre-push hook.
• Move the free-text credential redaction helper into gitremote.

Fixes #2986