docs(contributing): add CLA reference section to root guide

[biefan]

Summary

• add a Contributor License Agreement (CLA) section near the top of CONTRIBUTING.md
• include the official CLA portal link and CLA bot behavior (automatic check and one-time completion)
• place the section before Getting Started as requested

Testing

• npm run lint:md
• npm run spell-check

Resolves #237

[WilliamBerryiii]

Thank you for your interest in contributing to this project.

This PR is being closed for the following reasons:

1. PR template not used — All pull requests must use the repository's pull request template, which includes required sections for Description, Type of Change, Testing Performed, and a compliance Checklist.
2. Contribution guidelines not followed — Please review our Contributing Guide and Contribution Workflow before submitting PRs.
3. Insufficient validation — This PR was part of a batch of 22 PRs submitted within a 30-minute window without adequate local testing (e.g., required tooling like shellcheck/terraform was not available during PR creation).
4. Duplicate work — This issue is already being addressed by PR feat(build): add CLA section, Dependabot security prefix, and OWASP ZAP DAST scan #241.

If you'd like to contribute in the future, please fork the repository, follow the contribution workflow, use the PR template, and verify all acceptance criteria from the linked issue locally before submitting.

We appreciate your willingness to help and welcome future contributions that follow our project conventions.