docs(docs): add frontmatter to all sprint briefings

Standardized YAML frontmatter on all 115 sprint briefing files across
sprints 1-26 with title, description, type, status, sprint number, and
team fields.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/sprints/sprint-5/client.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 5 — Client Briefing"
+description: "Interaction prompt carry-over, UI microcopy joint work"
+type: sprint
+status: archived
+sprint: 5
+team: "client"
+---
+
 # Sprint 5: Live — Client Tasks
 
 **Goal:** Content at scale — FRIEND packs, PC-as-NPC authoring, NPC style guide