feat(voyage): bump trekbrief-template to brief_version 2.1 + add phase_signals fixtures

plugins/voyage/tests/fixtures/brief-without-phase-signals.md

@@ -0,0 +1,31 @@
+---
+type: trekbrief
+brief_version: "2.0"
+created: 2026-05-13
+task: "Backward-compat fixture for v5.0-style brief"
+slug: legacy-brief-example
+project_dir: .claude/projects/2026-05-13-legacy-brief-example/
+research_topics: 0
+research_status: complete
+auto_research: false
+interview_turns: 3
+source: interview
+---
+
+# Task: Legacy brief example
+
+## Intent
+
+A pre-v5.1 brief that pre-dates the phase_signals field. Used by
+tests/validators to confirm backward-compatibility: the brief is accepted
+without phase_signals as long as brief_version is < 2.1.
+
+## Goal
+
+Validator returns valid: true. The sequencing gate
+(BRIEF_V51_MISSING_SIGNALS) does NOT fire for brief_version 2.0.
+
+## Success Criteria
+
+- Validator passes.
+- No BRIEF_V51_MISSING_SIGNALS error in r.errors.