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/dashboard/src/__tests__/typecheck.test.ts

@@ -2,7 +2,7 @@ import { execFileSync } from "node:child_process";
 import { resolve } from "node:path";
 import { describe, it, expect } from "vitest";
 
-describe.skip("typecheck", () => {
+describe("typecheck", () => {
   it("passes tsc --noEmit --skipLibCheck false", () => {
     const cwd = resolve(__dirname, "../..");
     expect(() =>