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>

model-registry/minimax.yaml

@@ -0,0 +1,15 @@
+name: minimax
+endpoint: minimax
+provider: minimax
+scores:
+  complexity: 10
+  test_pass_rate: 9
+  creativity: 10
+  spec_adherence: 9
+  cost_efficiency: 3
+  context_utilisation: 10
+cost_per_1k_tokens:
+  input: 0.0
+  output: 0.0
+source: benchmark
+harness_version: null