feat(skills): add playwright-browser-test skill

[aguil]

Summary

Adds dot_agents/skills/playwright-browser-test/SKILL.md: a generic, vendor-agnostic skill for driving a browser with the Playwright CLI to verify UI changes end-to-end.

What it covers

• No-package.json setup (install @playwright/test to a temp dir)
• CommonJS unpack pattern for ESM scripts
• System Chrome via executablePath on macOS
• Persistent context to reuse login sessions across runs
• Login-flow detection gotcha — use a sign-in button, not input[type="password"] (hidden autofill fields cause false positives on the email step)
• File-injection MFA pattern: test polls /tmp/mfa-code.txt; injector writes the code from email/SMS automation
• OTP segmented inputs: keyboard.type() not fill()
• data-test-id (OverReact) vs data-testid (RTL/MUI): inspect the DOM first; some apps gate test attributes behind a URL flag

Context

Distilled from a live test session on a Dart/OverReact app.

Test plan

• Read SKILL.md for accuracy and completeness
• Verify code snippets are syntactically correct