chore: remove files that should not be in the repo

- README_PUSH.txt: private push instructions from a contributor,
  contained their GitHub username and Windows credential management
  steps — not appropriate for a public repo
- contracts/split/src/tests.rs: empty file (0 bytes), orphan duplicate
  of test.rs
- docs/IMPLEMENTATION_SUMMARY.md: internal PR summary note, not public
  documentation
- docs/STATUS_AND_NEXT_STEPS.md: internal planning note about a
  specific issue, not public documentation

Author: Kingsman-99