pipeline model routing: mechanical stages to MiniMax M3, spec stages to Fable low effort

- scaffolding-repo + test-writing-repo: anthropic-cloud-paul-oauth -> minimax + minimax-m3-max
- merge@1: new best-practices-minimax-repo composite (impl-review stays Sonnet)
- spec-draft-opus@1: Opus -> Fable 5 low effort via new fable-low-effort/v1
  env context (ANTHROPIC_MODEL=claude-fable-5, MAX_THINKING_TOKENS=1024) +
  spec-writing-fable-low-repo composite
- review-spec-arch-opus@1: Qwen3.6/airouter -> Fable 5 low effort via
  best-practices-fable-low-repo (arch review was on the cheapest model)

Template names kept (trigger rules reference them). Node model: fields
removed where env pins the model — a --model flag overrides ANTHROPIC_MODEL.

Claude-Session: https://claude.ai/code/session_019tJk7P8tZzJ24PvtgoGhLN

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

@@ -5,6 +5,9 @@ description: "Model-agnostic scaffolding harness — writes stubs for coding age
 
 layers:
   - context: scaffolding/v1
-  - context: anthropic-cloud-paul-oauth/v1
+  # 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: gitea-ssh/v1
   - context: agent-repo/v1