minimax: move to MiniMax-M3.1-Flash-Preview; drop bare minimax/minimax-m2 slugs

- new context minimax-m31-flash/v1 (model pin; no thinking-budget init, Flash
  ignores it); four composites switch from minimax-m3-max/v1 (kept for rollback)
- models + registry for minimax-m31-flash-{max,medium}; removes the dangling
  model-registry/minimax.yaml (endpoint "minimax" never existed)
- tiers, harness-roles, minimax-* task templates and the *-review-loop@1
  workflows use the Flash slugs (MiniMax silently maps unknown names to M3, so
  the provider-id translation lands in agent-runtimes)

model-registry/minimax-m31-flash-medium.yaml

@@ -0,0 +1,21 @@
+# agent-runtime-framework — model-registry/ (CRS tag: model-registry)
+# Medium-effort Flash; replaces minimax-m3 in the build/integrate/spec-test/coding tiers.
+# PLACEHOLDER scores/costs: scores are copied from the M3 entry this replaces and
+# costs are set just below it so cheapest-wins (tiers.yaml) selects Flash over
+# the retained M3 entries. Revisit with observed scores and published pricing.
+name: minimax-m31-flash-medium
+endpoint: minimax-m31-flash-medium
+provider: minimax
+family: minimax
+scores:
+  complexity: 8
+  test_pass_rate: 9
+  creativity: 8
+  spec_adherence: 9
+  cost_efficiency: 6
+  context_utilisation: 9
+cost_per_1k_tokens:
+  input: 0.00125
+  output: 0.00625
+source: benchmark
+harness_version: null