secure CI for fork and trusted pull requests

[erivaninan]

This PR updates the test workflow to handle fork and trusted pull requests separately.

• Fork pull requests run the test suite without repository secrets.
• Same-repository pull requests and trusted events retain access to OPENAI_API_KEY through the protected causal-agent environment.
• The workflow now uses pull_request instead of pull_request_target.
• actions/checkout is updated from v3 to v4.
• The workflow token is explicitly restricted to read-only repository access.

The previous workflow used pull_request_target, checked out code from a fork, and executed it with access to repository secrets. GitHub now blocks this pattern because it can lead to a "pwn request" vulnerability.

The new workflow preserves regular unit testing for fork contributions while keeping API-backed tests available only in trusted contexts. (This PR should be merged before PR#36 so that its pytest workflow can run safely.)