Include pack size estimates in subdivision notices

The "splitting N → M packs" notice told the user it happened but not how big the resulting packs would be. Surface the numbers we already have at each subdivision point:

• Pre-push (header estimate): now reads "estimated pack ~3.75 GB exceeds target limit 512 MB — splitting 1 → 8 packs (~480 MB each)". The estimate flows out of checkPackSizeAndSubdivide via its subdivide callback signature.

• Post-rejection: parses the body limit out of the server error and includes it: "target rejected pack (target limit 256 MB) — splitting 2 → 4 packs". We don't know the actual rejected pack size from the error, but the limit is the more actionable number anyway.

• One-shot → batched: already showed the new limit; left as-is.