Problem

The 128MB decompressed object size limit is hard-coded. Different deployments may need different limits — a code hosting service might want 50MB while a binary asset store might need 1GB.

Impact

• Cannot tune for specific use cases without code changes
• No per-repo or per-backend granularity
• Hard to document or communicate limits to users

Acceptance Criteria

• Object size limit configurable per-repo (via Repo.new options)
• Default remains 128MB for backward compatibility
• Limit enforced in: object decode, pack reader, receive-pack
• Clear error message when limit is exceeded, including the configured max
• Tests covering: custom limits, default behavior unchanged