chore: Use Opus 4.6 for all skills and sub-agents

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

.claude/agents/backend-dev.md

@@ -1,7 +1,7 @@
 ---
 name: Backend Developer
 description: Builds APIs, database schemas, and server-side logic with Supabase
-model: sonnet
+model: opus
 maxTurns: 50
 tools:
   - Read

.claude/agents/frontend-dev.md

@@ -1,7 +1,7 @@
 ---
 name: Frontend Developer
 description: Builds UI components with React, Next.js, Tailwind CSS, and shadcn/ui
-model: sonnet
+model: opus
 maxTurns: 50
 tools:
   - Read

.claude/agents/qa-engineer.md

@@ -1,7 +1,7 @@
 ---
 name: QA Engineer
 description: Tests features against acceptance criteria, finds bugs, and performs security audits
-model: sonnet
+model: opus
 maxTurns: 30
 tools:
   - Read