fix: address PR #194 review comments

- Delete plan files introduced in this PR (2026-03-09-*)
- Restore minimumvertexcover_to_qubo.json test data with new
  test_vc_to_qubo_ground_truth using MVC → MIS → SetPacking → QUBO path
- Fix test_find_cheapest_path_is_to_qubo: use Minimize("num_vars")
  instead of MinimizeSteps for deterministic path selection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: GiggleLiu