Add minimax model registry entry; rename tdd-impl workflow

- minimax.yaml: scores copied from Opus (placeholder until real benchmark),
  cost 0.0/0.0 (subscription), provider=minimax. Now auto-selectable by
  min_scores routing (smart-agent at complexity≥9 will reach it).
- tdd-impl → write-code-following-tdd (name field updated)

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

workflows/write-code-following-tdd.yaml

@@ -1,4 +1,4 @@
-name: tdd-impl
+name: write-code-following-tdd
 version: 1
 description: >
   Parallel TDD implementation: two agents implement against a pre-written test suite,