[BUG] README example: issue_comment trigger can run CLA workflow on non-PR issues

[AndrasZiegler]

Describe the bug
The README workflow example uses on: issue_comment with an if condition that checks only for the CLA magic phrase.
issue_comment events are fired for both pull requests and normal issues, so posting the phrase on a normal issue can run the CLA action without PR context.

To Reproduce

1. Configure the workflow as shown in the README (issue_comment + current if condition).
2. Open a normal GitHub Issue (not a PR).
3. Add this comment to the issue: I have read the CLA Document and I hereby sign the CLA.
4. Observe that the workflow is triggered for that issue comment.

Expected behavior
The CLA workflow should only run for:

• pull_request_target events, or
• issue_comment events where the comment is on a pull request (github.event.issue.pull_request exists).

Non-PR issue comments should not trigger CLA processing.

Screenshots
N/A