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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "promptarch",
-  "version": "1.0.0",
+  "version": "1.3.0",
   "description": "Transform vague ideas into production-ready prompts and PRDs",
   "scripts": {
     "dev": "next dev",