Add lightweight portfolio CI baseline - #14

Summary

• add a least-privilege GitHub Actions workflow for structural validation and all ten project test suites
• validate Python and notebook code syntax, local Markdown links, whitespace, and secret/local-path patterns
• document the intentionally lightweight CI scope and preserve Pima's scikit-learn upper bound

Local validation

• 76 tests passed across ten isolated project suites
• 69 Python files parsed
• 10 notebooks and 28 code cells parsed without notebook execution
• 34 local Markdown links passed
• 104 text files passed the sensitive/local-path guard
• git diff --check passed

CI uses Python 3.11, read-only contents permission, no secrets, no production access, and no deployment. Optional research frameworks, datasets/models, notebook execution, GPU work, model retraining, and result regeneration are deliberately excluded.