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-1/client.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 1 — Client Briefing"
+description: "MessagePack GDScript serialization, LocalBridge GDScript implementation"
+type: sprint
+status: archived
+sprint: 1
+team: "client"
+---
+
 # Sprint 1: Run — Client Tasks
 
 **Goal:** Get a player-controlled character moving on screen through working IPC bridge.

docs/sprints/sprint-1/joint.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 1 — Joint Briefing"
+description: "End-to-end connection test, tick loop synchronization, moving character proof"
+type: sprint
+status: archived
+sprint: 1
+team: "joint"
+---
+
 # Sprint 1: Run — Joint Tasks
 
 **Goal:** Get a player-controlled character moving on screen through working IPC bridge.

docs/sprints/sprint-1/server.md

@@ -1,3 +1,12 @@
+---
+title: "Sprint 1 — Server Briefing"
+description: "LocalBridge Rust implementation, tile collision system, IPC protocol"
+type: sprint
+status: archived
+sprint: 1
+team: "server"
+---
+
 # Sprint 1: Run — Server Tasks
 
 **Goal:** Get a player-controlled character moving on screen through working IPC bridge.