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/client.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 3 — Client Briefing"
+description: "Interaction prompt system, client-side interaction infrastructure"
+type: sprint
+status: archived
+sprint: 3
+team: "client"
+---
+
 # Sprint 3: Content Foundation — Client Tasks
 
 **Goal:** Build client-side interaction infrastructure to support v0.1 gameplay.

docs/sprints/sprint-3/copy.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 3 — Copy Briefing"
+description: "Character specs, NPC patterns, content directory structure design, wiki maintenance"
+type: sprint
+status: archived
+sprint: 3
+team: "copy"
+---
+
 # Sprint 3: Content Foundation — Copy Tasks
 
 **Goal:** Build the content design foundation for v0.1 — character specs, patterns, content structure, and wiki maintenance.

docs/sprints/sprint-3/joint.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 3 — Joint Briefing"
+description: "Cross-team content infrastructure coordination, copy-to-server dependency chain"
+type: sprint
+status: archived
+sprint: 3
+team: "joint"
+---
+
 # Sprint 3: Content Foundation — Joint Tasks
 
 **Goal:** Coordinate cross-team dependencies for content infrastructure and interaction systems.

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.