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
- AGENTS.md and README updated

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

.opencode/skills/development-guidelines/SKILL.md

@@ -89,6 +89,7 @@ When creating development guidelines: ./template.md
 ### When Designing Tests
 - "Test Strategy" in ./guides/process.md (pyramid, coverage)
 - "Test Code" in ./guides/implementation.md (implementation patterns)
+- E2E tests: the **e2e-testing** skill (Playwright — E2E is part of every verification step)
 
 ### When Preparing a Release
 - "Git Workflow Rules" in ./guides/process.md (policy for merging into main)