Summary

• Add per-repo max_object_size option to Repo.new/2 (default 128MB unchanged)
• Enforce limit in object decode, pack reader (via ObjectResolver), and receive-pack
• Error messages include the configured limit for clarity
• Update CHANGELOG and README for recent features (fsck, dialyzer)

Closes #19

Changes

• lib/ex_git_objectstore/repo.ex — New max_object_size field with 128MB default
• lib/ex_git_objectstore/object.ex — decode/2 accepts max_size: option; read/2 uses repo limit; error tuple simplified to {:object_too_large, detail_string}
• lib/ex_git_objectstore/object_resolver.ex — Pack object size check against repo limit
• lib/ex_git_objectstore/protocol/receive_pack.ex — Incoming push object size validation
• CHANGELOG.md — Unreleased section with recent additions
• README.md — Added fsck to features list

Test plan

• Default max_object_size is 128MB
• Custom limit stored in repo struct
• Object.read rejects objects exceeding custom limit
• Object.decode respects custom max_size option
• Error message includes the configured limit
• All 547 tests pass, dialyzer clean