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/agents/implementation-validator.md

@@ -53,6 +53,7 @@ Verify that the implemented code meets the following criteria:
 - [ ] Is the coverage target met?
 - [ ] Are edge cases tested?
 - [ ] Are the tests named appropriately?
+- [ ] Are there Playwright E2E tests covering the feature's acceptance criteria (e2e/*.spec.ts)?
 
 **Evaluation Criteria**:
 - ✅ Sufficient: Coverage of 80% or more, covering the main cases
@@ -213,6 +214,11 @@ npm test
 npm run test:coverage
 ```
 
+### Run E2E Tests (Playwright)
+```bash
+npm run test:e2e
+```
+
 ### Build Check
 ```bash
 npm run build