fix(git-pilot): replace diffstat with file count in summary

[moukrea]

Summary

• Replaces the verbose per-file git diff --stat output in the session-stop summary with a single "Files changed: N" line
• Keeps the commit list intact (compact and useful)
• Reduces token waste and improves visibility of Claude's actual answers at session end

Test plan

• End a session with commits and verify the summary shows "Files changed: N" instead of the full diffstat
• Verify commit list still appears correctly in the summary
• Verify sessions with no changes still produce no summary