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-15/server.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 15 — Server Briefing"
+description: "Monologue event generation, NPC generation pipeline, follow mechanic"
+type: sprint
+status: archived
+sprint: 15
+team: "server"
+---
+
 # Sprint 15: React — Server Tasks
 
 **Goal:** The player acts and the world reacts — follow and examine mechanics connect the player to the living NPC simulation; the dialogue system's first two access layers open up information gating; server-side monologue events fire in context; and the personality/tell system completes the NPC data model.