test(validators): cover fable accept + unknown-model reject in both gate layers

tests/fixtures/brief-effort-fable.md

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+---
+type: trekbrief
+brief_version: "2.1"
+created: 2026-07-02
+task: "Fixture: high-effort all phases on fable (v5.9 allowlist test)"
+slug: brief-effort-fable
+project_dir: .claude/projects/2026-07-02-brief-effort-fable/
+research_topics: 0
+research_status: complete
+auto_research: false
+interview_turns: 4
+source: fixture
+phase_signals:
+  - phase: research
+    effort: high
+    model: fable
+  - phase: plan
+    effort: high
+    model: fable
+  - phase: execute
+    effort: high
+    model: fable
+  - phase: review
+    effort: high
+    model: fable
+---
+
+# Task: High-effort fable fixture
+
+## Intent
+
+Test fixture for the v5.9 fable model tier — all 4 phases at the
+high effort tier with explicit fable model overrides. Mirrors
+brief-effort-high.md with `model: opus` replaced by `model: fable`.
+
+## Goal
+
+Resolver returns `{effort: 'high', model: 'fable'}` for each of the 4
+PHASE_SIGNAL_PHASES.
+
+## Success Criteria
+
+- Validator passes with no BRIEF_INVALID_MODEL.
+- resolvePhaseSignal(fm, phase).effort === 'high' for all 4 phases.
+- resolvePhaseSignal(fm, phase).model === 'fable' for all 4 phases.