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/tiers.yaml

@@ -22,7 +22,7 @@ tiers:
       context_utilisation: 9
     qualifies_today:
       - deepseek-v4flash       # benchmark-inflated 9s; hold to allowed_models ceiling
-      - minimax
+      - minimax-m31-flash-max
       - claude-opus-5
     note: >
       WF-SCH-14 allowed_models ceiling remains authoritative until M7 observed-score
@@ -37,7 +37,8 @@ tiers:
       spec_adherence: 9
       test_pass_rate: 9
     qualifies_today:
-      - minimax
+      - minimax-m31-flash-max
+      - minimax-m31-flash-medium
       - claude-sonnet-5
       - claude-opus-5
 
@@ -51,7 +52,8 @@ tiers:
     qualifies_today:
       - airouter-qwen3
       - airouter-deepseekv4flash
-      - minimax
+      - minimax-m31-flash-max
+      - minimax-m31-flash-medium
       - claude-sonnet-5
       - claude-opus-5
       - claude-fable-5