feat: v1.3.0 — plan-first workflow, OpenRouter provider, enhanced prompt engine

Major changes:
- Plan-first workflow: AI generates structured plan before code, with
  plan review card (Modify Plan / Start Coding / Skip to Code)
- Post-coding UX: Preview + Request Modifications buttons after code gen
- OpenRouter integration: 4th AI provider with 20+ model support
- Enhanced prompt engine: 9 strategies, 11+ intent patterns, modular
- PLAN MODE system prompt block in all 4 services
- Fixed stale React closure in approveAndGenerate with isApproval flag
- Fixed canvas auto-opening during plan phase with wasIdle gate
- Updated README, CHANGELOG, .env.example, version bump to 1.3.0

.env.example

@@ -15,6 +15,11 @@ ZAI_API_KEY=
 ZAI_GENERAL_ENDPOINT=https://api.z.ai/api/paas/v4
 ZAI_CODING_ENDPOINT=https://api.z.ai/api/coding/paas/v4
 
+# OpenRouter API
+# Get API key from https://openrouter.ai/keys
+OPENROUTER_API_KEY=
+OPENROUTER_DEFAULT_MODEL=google/gemini-2.0-flash-exp:free
+
 # Site Configuration (Required for OAuth in production)
 # Set to your production URL (e.g., https://your-app.vercel.app)
 NEXT_PUBLIC_SITE_URL=http://localhost:6002