docs(docs): add frontmatter to all design docs

Standardized YAML frontmatter on all 40 docs/design/ files with title,
description, type, status, ticket, decision_refs, and author fields.
Covers specs, wireframes, voice cards, spatial layouts, and system
designs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/design/content-structure-canonical.md

@@ -1,3 +1,15 @@
+---
+title: "Content Structure — Canonical Template (v0.2+)"
+description: "Post-cleanup canonical directory layout for campaign content, superseding v0.1 content-directory-structure"
+type: design
+status: active
+ticket: "#656"
+decision_refs: [D-117, D-138]
+author: ""
+created: 2026-03-13
+updated: 2026-03-13
+---
+
 # Content Structure — Canonical Template (v0.2+)
 
 **Created:** 2026-03-13 (Sprint 26, #656 cleanup)