feat(voyage)!: rename type discriminators across validators + fixtures [skip-docs]

- brief-validator: BRIEF_TYPE_VALUES ['ultrabrief','ultrareview'] -> ['trekbrief','trekreview'] + dependent branches
- research-validator: 'ultraresearch-brief' -> 'trekresearch-brief'
- review-validator: 'ultrareview' -> 'trekreview'
- 3 templates frontmatter type:
- 4 synthetic fixtures: ultraplan-synthetic/ultrareview-synthetic -> trek* (frontmatter only; bodies untouched, Jaccard floor preserved)
- 2 trekreview fixtures: type: trekreview
- 6 validator-test fixtures + asserts
- agents/review-coordinator.md frontmatter example

Atomic: validator + fixtures committed together — partial state would cause vacuous
test passes or hard validator rejection.

Part of voyage-rebrand session 2 (W3.3 / Step 5).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/ultraplan-local/tests/fixtures/trekreview/review-run-A.md

@@ -1,5 +1,5 @@
 ---
-type: ultrareview
+type: trekreview
 review_version: "1.0"
 created: 2026-05-01
 task: "Add JWT authentication with refresh-token rotation"

plugins/ultraplan-local/tests/fixtures/trekreview/review-run-B.md

@@ -1,5 +1,5 @@
 ---
-type: ultrareview
+type: trekreview
 review_version: "1.0"
 created: 2026-05-01
 task: "Add JWT authentication with refresh-token rotation"