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)

models/minimax-m31-flash-max.yaml

@@ -0,0 +1,21 @@
+# agent-runtime-framework — models/ (CRS tag: models)
+# MiniMax-M3.1-Flash-Preview at reasoning_effort max. Always thinks (thinking
+# cannot be disabled; effort is the only depth control). The claude runner maps
+# default_flags.reasoning_effort to `--effort` (agent-runtimes ME-6a). Verified
+# 2026-10-13: the container MiniMax key reaches this model on both
+# api.minimax.io and api.minimaxi.chat; 1M context window.
+# Endpoint name MUST equal the registry name (MR-12).
+name: minimax-m31-flash-max
+provider: minimax
+family: minimax
+endpoint:
+  type: cloud_api
+  base_url: "https://api.minimax.io/anthropic"
+  api_key_env: MINIMAX_API_KEY
+runner: claude
+default_flags:
+  model: "MiniMax-M3.1-Flash-Preview"
+  reasoning_effort: max
+provider_context_ref: "minimax/v1"
+provider_secret_ref: "MINIMAX_API_KEY"
+session_capable: false

models/minimax-m31-flash-medium.yaml

@@ -0,0 +1,21 @@
+# agent-runtime-framework — models/ (CRS tag: models)
+# MiniMax-M3.1-Flash-Preview at reasoning_effort medium. Always thinks (thinking
+# cannot be disabled; effort is the only depth control). The claude runner maps
+# default_flags.reasoning_effort to `--effort` (agent-runtimes ME-6a). Verified
+# 2026-10-13: the container MiniMax key reaches this model on both
+# api.minimax.io and api.minimaxi.chat; 1M context window.
+# Endpoint name MUST equal the registry name (MR-12).
+name: minimax-m31-flash-medium
+provider: minimax
+family: minimax
+endpoint:
+  type: cloud_api
+  base_url: "https://api.minimax.io/anthropic"
+  api_key_env: MINIMAX_API_KEY
+runner: claude
+default_flags:
+  model: "MiniMax-M3.1-Flash-Preview"
+  reasoning_effort: medium
+provider_context_ref: "minimax/v1"
+provider_secret_ref: "MINIMAX_API_KEY"
+session_capable: false