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/spec-writing-fable-low-repo/v1/harness.yaml

@@ -0,0 +1,11 @@
+kind: composite
+name: spec-writing-fable-low-repo
+version: 1
+description: "Spec writing agent on Fable 5 (low effort) + repo clone via SSH"
+
+layers:
+  - context: spec-writing/v1
+  - context: anthropic-cloud-paul-oauth/v1
+  - context: fable-low-effort/v1
+  - context: gitea-ssh/v1
+  - context: agent-repo/v1