Recombine checkpoints when consecutive packs underuse the target limit

Subdivision is a one-way ratchet: the fine granularity needed to fit one heavy commit through the limit sticks for the rest of the chain, leading to thousands of tiny pushes after the heavy region has passed. cli-checkpoints reproduces this — one ~30 MB commit forces 928 → 7967 splits, but the commits behind it are 6-object deltas that comfortably fit dozens per pack.

After every successful push, drop enough upcoming checkpoints that the next pack should land near half the limit. Each dropped checkpoint roughly doubles the next pack's span, so the count is log2(target/2 / sent), capped to keep recovery cost bounded if a heavy commit shows up immediately after. If we overshoot, the existing abort-early plus subdivision path re-splits.