Require Playwright E2E tests in every verification step

- New skill e2e-testing: on-demand Playwright setup (config, test:e2e script), acceptance-criteria-driven specs (one per feature + app-shell smoke), blueprint-driven semantic locators, and the shared verification checklist
- /add-feature Step 7 and /execute-milestones Step 2.6 now write/update E2E specs and run npm run test:e2e alongside test/lint/typecheck; milestone gates and completion criteria include E2E
- /generate-app Step 5 sets up Playwright and creates the app-shell smoke spec for the web target (integration_test / UI automation noted for flutter/winui3)
- steering tasklist template, milestone-planning Definition of Done, milestone-execution task slicing/gates, development-guidelines pointers, and implementation-validator checks updated
- CLAUDE.md, README, and settings.json updated

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.claude/skills/steering/templates/tasklist.md

@@ -59,6 +59,9 @@ When skipping, always state the reason clearly:
   - [ ] `npm run lint`
 - [ ] Confirm that there are no type errors
   - [ ] `npm run typecheck`
+- [ ] Confirm that the E2E tests pass (Playwright — see the e2e-testing skill)
+  - [ ] E2E specs exist for this work's acceptance criteria
+  - [ ] `npm run test:e2e`
 - [ ] Confirm that the build succeeds
   - [ ] `npm run build`