test: add MEDIUM priority rebase e2e tests (M1, M3, M5, M7, M8)

Add 5 integration tests covering all remaining MEDIUM priority gaps:
- Step mode + squash-merge Reset interaction (#41)
- Step mode + ignore-root flag (#42)
- Continue when git rebase still in progress (#43)
- Skip with --cleanup-backups flag (#44)
- Progress reporting during continue (#45)

Author: dashed