Visual QA Checkpoint

Lightweight visual smoke test for UI-facing changes.

Writes result to .checkpoints/visual-qa.json.

IMPORTANT: Not for Workers on Worktrees

This checkpoint should only be run by the conductor AFTER merging changes to main.

Workers on git worktrees cannot run visual-qa because:

1. Changes aren't built - The extension/app isn't rebuilt with worktree changes
2. No isolated browser - Workers share the same Chrome instance
3. Tab group conflicts - Multiple workers fighting for tabs (especially if groups disabled)
4. Dev server conflicts - Multiple npm run dev instances on same port