fix(voyage): coerce brief_version to string + quote template + update doc pin (closes #8 #11)

v5.1.0 shipped with an unquoted brief_version: 2.1 in trekbrief-template.md.
parseScalar coerced it to Number 2.1, and the sequencing gate guarded on
typeof === 'string', silently bypassing BRIEF_V51_MISSING_SIGNALS.

Three-part atomic fix:
- brief-validator.mjs:87+149 now accepts both string and number forms via
  String(fm.brief_version) coercion.
- trekbrief-template.md quotes the value so new briefs parse as String.
- doc-consistency.test.mjs pins the QUOTED form going forward.

Three regression tests added in brief-validator.test.mjs.

plugins/voyage/templates/trekbrief-template.md

@@ -1,6 +1,6 @@
 ---
 type: trekbrief
-brief_version: 2.1
+brief_version: "2.1"
 created: {YYYY-MM-DD}
 task: "{one-line task description}"
 slug: {slug}