TEST deletion of AI lines in the next commit

[AtnesNess]

#193

---

Note

Adds regression tests for correct attribution when AI deletes its own lines and introduces detailed checkpoint debug logging.

• Tests:
  • Add e2e test tests/e2e/user-scenarios.bats verifying stats when AI deletes one of its own lines and both human and AI add lines in the same commit (human_additions: 2, ai_additions: 2, total_ai_deletions: 1).
  • Add Rust test tests/simple_additions.rs::test_ai_deletion_with_human_checkpoint_in_same_commit asserting correct line-by-line blame and stats for the same scenario.
• Debugging:
  • Add verbose println! debugging in src/commands/checkpoint.rs within get_checkpoint_entry_for_file and make_entry_for_file to log checkpoint kind, author, data source, previous/current content, previous/updated attributions, line attributions, and skip reasons.

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[svarlamov]

Closing this PR as I've added the test in ba35c5c

as part of the work in #228