Fix diff parsing logic to correctly extract file paths and line counts

[oleander]

Task

Fix the diff parsing logic in src/multi_step_integration.rs to correctly handle standard git diff formats.

Requirements

• Accurately extract file paths from git diff headers by recognizing the diff --git a/path b/path format
• Strip the conventional a/ and b/ prefixes that git uses
• Handle special cases:
  • Newly added files (old path is /dev/null)
  • Deleted files (new path is /dev/null)
  • Renamed files (capture both paths)
• Correct line counting:
  • Lines beginning with + are additions
  • Lines beginning with - are removals
  • Exclude metadata lines (+++ or ---)
• Infer file structure from available context when headers are missing
• Only default to "unknown" filename after exhausting all parsing strategies

Success Criteria

• Parser successfully extracts correct file paths from standard git diff output
• Line counts accurately reflect additions and removals
• No more "unknown" filename issues in debug output
• File paths appear as actual repository paths, not generic placeholders

Part of #81