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)

harnesses/composites/best-practices-minimax-repo/v1/harness.yaml

@@ -6,6 +6,6 @@ description: "Best practices review agent on MiniMax M3 (max effort) + repo clon
 layers:
   - context: best-practices/v1
   - context: minimax/v1
-  - context: minimax-m3-max/v1
+  - context: minimax-m31-flash/v1
   - context: gitea-ssh/v1
   - context: agent-repo/v1

harnesses/composites/integration-direct/v1/harness.yaml

@@ -9,7 +9,7 @@ layers:
   # personal-subscription OAuth — integration runs are frequent/mechanical
   # and were draining the interactive 5-hour window (2026-09-02).
   - context: minimax/v1
-  - context: minimax-m3-max/v1
+  - context: minimax-m31-flash/v1
   - context: gitea-ssh/v1
   - context: agent-repo/v1
   - context: direct-push/v1

harnesses/composites/scaffolding-repo/v1/harness.yaml

@@ -8,6 +8,6 @@ layers:
   # MiniMax M3 — scaffolding is mechanical (stub files so tests import);
   # moved off the personal Anthropic subscription 2026-09-02.
   - context: minimax/v1
-  - context: minimax-m3-max/v1
+  - context: minimax-m31-flash/v1
   - context: gitea-ssh/v1
   - context: agent-repo/v1

harnesses/composites/test-writing-repo/v1/harness.yaml

@@ -9,6 +9,6 @@ layers:
   # 2026-09-02 (the "credential layer swappable" swap this composite was
   # designed for).
   - context: minimax/v1
-  - context: minimax-m3-max/v1
+  - context: minimax-m31-flash/v1
   - context: gitea-ssh/v1
   - context: agent-repo/v1