test(evals): Voyage's fixed test set as a plugin eval suite, expectations written before the first run

Seven `claude plugin eval` cases under evals/, runs: 1, deterministic graders
only (regex, tool_used, file_exists), no ablation. Each case tests one thing
Voyage promises and stops within minutes, headless:

- plan-requires-brief, plan-project-not-initialized, plan-rejects-unknown-export,
  review-requires-project: argument guards stop before any Agent or Write.
- plan-halts-without-phase-signals: a brief_version 2.1 brief without
  phase_signals halts /trekplan at the sequencing gate, before the swarm.
- review-validate-flags-bad-finding-id (known-positive) and
  review-validate-passes-clean-review (known-negative): /trekreview --validate
  names REVIEW_BAD_FINDING_ID on a planted bad ID and stays clean on a valid file.

Each case's expected_outcome is committed here, before the suite has run.
Fixtures are written by an inline scaffold.sh (needs --scaffold): chose inline
heredocs because a run cannot read the eval directory, and the add_dirs path
mapping is not documented. evals/results/ is ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

evals/plan-requires-brief/graders/no-agent.md

@@ -0,0 +1,6 @@
+---
+type: tool_used
+tool: Agent
+min: 0
+max: 0
+---

evals/plan-requires-brief/graders/no-write.md

@@ -0,0 +1,6 @@
+---
+type: tool_used
+tool: Write
+min: 0
+max: 0
+---

evals/plan-requires-brief/graders/usage-stop.md

@@ -0,0 +1,4 @@
+---
+type: regex
+pattern: "A brief is required"
+---