feat: improve prompt context with file operation categories

[vavasilva]

Summary

• Added file categorization (added, deleted, modified) to DiffResult type
• Updated getStagedDiff, getFileDiff, and getLastCommitDiff to detect file operations
• Updated context building to show categorized files, helping LLM generate better commit messages

This improves commit message quality for file deletions and renames by giving the LLM explicit context about what operations were performed.

Test plan

• All tests pass
• Build succeeds
• Manual test: delete a file and verify commit message is appropriate (e.g., "chore: remove file" instead of "fix: update file")

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