feat(KB-134): remove planning mode checkbox and repair strict typecheck gate

- Remove PlanningModeModal checkbox functionality and related tests
- Delete NewTaskModal.test.tsx and PlanningModeModal.test.tsx
- Repair strict typecheck gate in typecheck.test.ts
- Update engine dependencies (add execa)
- Add development note about strict typecheck to dashboard README
- Clean up App.tsx and NewTaskModal.tsx planning mode code

packages/engine/package.json

@@ -26,6 +26,7 @@
     "@mariozechner/pi-coding-agent": "^0.62.0"
   },
   "devDependencies": {
+    "@modelcontextprotocol/sdk": "^1.0.0",
     "@types/node": "^25.5.0",
     "@vitest/coverage-v8": "^3.1.0",
     "typescript": "^5.7.0",