Add behavioral trust badge to README

[vdineshk]

Suggestion

Add a Dominion Observatory behavioral trust badge to the README. This gives users a quick visual indicator of the server's trust score.

Badge code:

![Trust Score](https://dominionobservatory.com/badge/mcp-playwright)

We also have a GitHub Action that auto-scans MCP configs in PRs and posts trust scores as comments:

- uses: vdineshk/dominion-observatory/github-action@main
  with:
    threshold: 50

No API key needed. 14,824 servers scored.