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-3/server.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 3 — Server Briefing"
+description: "Content directory skeleton, YAML schema definitions, validate-content CLI, proximity detection"
+type: sprint
+status: archived
+sprint: 3
+team: "server"
+---
+
 # Sprint 3: Content Foundation — Server Tasks
 
 **Goal:** Build the content infrastructure for v0.1 — directory structure, schema validation, and interaction mechanics.