feat: add `solve_by_elim +suggestions` (#11468)

This PR adds `+suggestions` support to `solve_by_elim`, following the
pattern established by `grind +suggestions` and `simp_all +suggestions`.

Gracefully handles invalid/nonexistent suggestions by filtering them out

🤖 Prepared with Claude Code

Co-authored-by: Claude <noreply@anthropic.com>
3 files changed