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/templates/research-brief-template.md

@@ -1,5 +1,5 @@
 ---
-type: ultraresearch-brief
+type: trekresearch-brief
 created: {YYYY-MM-DD}
 question: "{research question}"
 confidence: {0.0-1.0}

plugins/ultraplan-local/templates/trekbrief-template.md

@@ -1,5 +1,5 @@
 ---
-type: ultrabrief
+type: trekbrief
 brief_version: 2.0
 created: {YYYY-MM-DD}
 task: "{one-line task description}"

plugins/ultraplan-local/templates/trekreview-template.md

@@ -1,5 +1,5 @@
 ---
-type: ultrareview
+type: trekreview
 review_version: "1.0"
 created: {YYYY-MM-DD}
 task: "{Task description from brief.md}"