Add CI troubleshooting notes for Playwright browser setup

[rxdt]

Goal

Document the common CI failure where Playwright tests use multiple browser projects but CI installs only one browser.

Scope

• Add a short troubleshooting note in README or docs.
• Mention that pnpm --prefix harness run setup:e2e installs the browser set expected by the Playwright config.
• Keep it focused on maintainers running this template in GitHub Actions.

Acceptance criteria

• The note explains the symptom and the fix.
• The change is documentation-only.
• pnpm gate passes.