Add GitHub Actions workflow for PR validation

[travisjneuman]

Description

Add a GitHub Actions workflow that runs on pull requests to validate contributions automatically. This should run the curriculum validation script, check code formatting, and run any tests.

Acceptance Criteria

• Create .github/workflows/pr-validation.yml
• Run python scripts/validate_curriculum.py on every PR
• Run ruff check . on changed Python files
• Report results clearly in the PR checks
• Keep the workflow fast (under 2 minutes)

[travisjneuman]

Resolved in 36dceb1: Added .github/workflows/pr-validation.yml — lightweight PR validation with ruff, py_compile, root doc, level index, elite track, and modality hub checks.