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/contexts/fable-low-effort/v1/harness.yaml

@@ -0,0 +1,16 @@
+kind: context
+name: fable-low-effort
+version: 1
+description: "Pin Claude Fable 5 at low reasoning effort — layer AFTER anthropic-cloud-paul-oauth/v1 (auth comes from there)"
+requires: [claude-code]
+provides: []
+
+# Model pin + effort dial for Claude Code against the Anthropic cloud.
+# - ANTHROPIC_MODEL forces Fable 5; workflow nodes using this layer must
+#   NOT set a node-level model: (the --model flag would override this env).
+# - MAX_THINKING_TOKENS keeps the extended-thinking budget small ("low
+#   effort") — Fable's raw capability at minimal thinking is the point:
+#   frontier judgment without frontier token burn.
+env:
+  ANTHROPIC_MODEL: "claude-fable-5"
+  MAX_THINKING_TOKENS: "1024"