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)

task-templates/workflow/test-write-review-loop@1.yaml

@@ -21,7 +21,7 @@ nodes:
     role: test-write-review  # M36 v0.6 (bug 69370eda fix): role + model replaces
                              # the ungrounded `template: minimax-test-writer`
                              # (a composite that never existed in the catalog).
-    model: minimax-m2
+    model: minimax-m31-flash-max
     runtime_env:
       # WF-COMP-15: stage agents share one per-item work branch so their work
       # accumulates across iterations.