feat: add @playwright/cli to agent container image

Pre-install @playwright/cli and Chromium headless shell in the AWF
agent container, replacing the separate Playwright MCP Docker
container approach. Update smoke-claude and smoke-codex to use
playwright-cli bash commands. Add docs for the new approach.

Agent-Logs-Url: https://github.com/github/gh-aw-firewall/sessions/0b42f528-4a25-44a3-9b05-90864c36ffd2

Author: Copilot