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/milestone-planning/template.md

@@ -59,7 +59,8 @@ Use this structure for each `docs/milestones/phase[N]-milestones.md`. A phase fi
 ### Definition of Done
 
 - [ ] All features above are checked off
-- [ ] `npm test`, `npm run lint`, `npm run typecheck` pass
+- [ ] Every feature has a Playwright E2E spec covering its acceptance criteria
+- [ ] `npm test`, `npm run lint`, `npm run typecheck`, `npm run test:e2e` pass
 - [ ] Persistent documents in `docs/` are reconciled with all features
 - [ ] [Milestone-specific criterion, e.g. "demo flow X→Y→Z works end-to-end"]