Fix rebase conflict resolution attribution

[svarlamov]

Summary

• record explicit checkpoints for conflicted files instead of dropping them while the index is unmerged
• merge conflict-resolution working-log attribution into the already-shifted rebased commit note before writing the note
• recover exact kept source lines from immutable source/destination commit trees so keep-main, keep-feature, and keep-both resolutions preserve the matching original attribution
• add TestRepo coverage for AI-resolved conflicts and all keep-side conflict resolution shapes

Root Cause

During rebase conflicts, checkpointing skipped conflicted files, and the post-rebase resolution path treated the conflict-resolution working log separately from the shifted source note. That meant resolutions either lost the new checkpoint attribution or overwrote source attribution instead of combining the immutable commit data and the persisted working log in one deterministic path.

Validation

• task fmt
• task lint
• git diff --check
• task test

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.