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-2/client.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 2 — Client Briefing"
+description: "Camera lock, tile rendering engine, fog overlay rendering, entity sprite management"
+type: sprint
+status: archived
+sprint: 2
+team: "client"
+---
+
 # Sprint 2: See — Client Tasks
 
 **Goal:** Fog of perception working through the bridge — server computes LOS visibility, client renders fog.