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

task-templates/workflow/merge@1.yaml

@@ -1,7 +1,9 @@
 name: merge
 version: 1
 description: Prepare a reviewed implementation for merge — runs pre-merge checks and marks item merge-ready
-runtime: best-practices-opus-repo
+# MiniMax M3 (2026-09-02): merge prep is mechanical (checklist + verdict);
+# moved off the Anthropic subscription. impl-review stays on Sonnet.
+runtime: best-practices-minimax-repo
 input:
   state: impl-reviewed
   tags_forbidden: []
@@ -15,8 +17,7 @@ scope_budget:
 nodes:
   - id: agent
     kind: agent
-    template: best-practices-opus-repo
-    model: sonnet
+    template: best-practices-minimax-repo
     runtime_env:
       AGENT_REPO_URL: "git@gitea.oreillyit.nz-ai-enablement:skynet/agent-runtimes-agents.git"
       AGENT_BRANCH: "task/${item.uuid}"