test(voyage): add 4 brief fixtures for v5.1.1 runtime scenarios

plugins/voyage/tests/fixtures/brief-effort-low.md

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+---
+type: trekbrief
+brief_version: "2.1"
+created: 2026-05-14
+task: "Fixture: low-effort all phases (v5.1.1 runtime test)"
+slug: brief-effort-low
+project_dir: .claude/projects/2026-05-14-brief-effort-low/
+research_topics: 0
+research_status: complete
+auto_research: false
+interview_turns: 4
+source: fixture
+phase_signals:
+  - phase: research
+    effort: low
+    model: sonnet
+  - phase: plan
+    effort: low
+    model: sonnet
+  - phase: execute
+    effort: low
+    model: sonnet
+  - phase: review
+    effort: low
+    model: sonnet
+---
+
+# Task: Low-effort fixture
+
+## Intent
+
+Test fixture for v5.1.1 runtime resolver tests — all 4 phases at the lowest
+effort tier with explicit sonnet model overrides.
+
+## Goal
+
+Resolver returns `{effort: 'low', model: 'sonnet'}` for each of the 4
+PHASE_SIGNAL_PHASES.
+
+## Success Criteria
+
+- Validator passes.
+- resolvePhaseSignal(fm, phase) is non-null for all 4 phases.