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-21/server.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 21 — Server Briefing"
+description: "Template definition format, template-to-instance mapping, cross-template triangle generation"
+type: sprint
+status: archived
+sprint: 21
+team: "server"
+---
+
 # Sprint 21: Instantiate — Server Tasks
 
 **Goal:** The template system becomes executable — templates spawn NPCs, assign triangles, and place them in world space; cross-template triangles link social sites; the client gains save/load game flow; and the generator pipeline gets its architectural design.