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/spec-draft-opus@1.yaml

@@ -1,7 +1,10 @@
 name: spec-draft-opus
 version: 1
 description: Draft a spec from an idea using spec-writer agent
-runtime: spec-writing-opus-repo
+# Fable 5 low effort (2026-09-02) — template name kept for trigger compat.
+# Model comes from the fable-low-effort/v1 env layer; no node model: here
+# (a --model flag would override ANTHROPIC_MODEL).
+runtime: spec-writing-fable-low-repo
 input:
   state: idea
   tags_required: []
@@ -16,8 +19,7 @@ scope_budget:
 nodes:
   - id: agent
     kind: agent
-    template: spec-writing-opus-repo
-    model: opus
+    template: spec-writing-fable-low-repo
     runtime_env:
       AGENT_REPO_URL: "git@gitea.oreillyit.nz-ai-enablement:skynet/agent-runtimes-agents.git"
       AGENT_BRANCH: "spec/${item.uuid}"