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/minimax-test-writer.yaml

@@ -1,6 +1,6 @@
 name: minimax-test-writer
 description: "MiniMax test writing agent — comprehensive tests with edge cases"
-model: minimax
+model: minimax-m31-flash-max
 harness: test-writing-minimax-repo/v1
 requires_tags: [claude-code, test-writer, best-practices, git-access, project-repo]
 required_params: [repo_url, agent_repo_url]