ci: run tests on pull requests

[x1f9-ricon]

Summary

• add the standard KKL GitHub Actions test workflow
• run mise run test on Ubuntu and macOS

Verification

• mise run test
• CALLER_PWD="$PWD" mise run -q --cd ~/agents/x1f9/codebase lint:github-actions "$PWD"

[junior-ricon]

Reviewed the generated workflow. It matches the standard KKL test workflow, actionlint accepts it, git diff --check origin/main...HEAD is clean, local mise run test passes (20 tests), and both GitHub matrix jobs are green.

[brownie-ricon]

No findings. The generated workflow is the standard KKL test workflow and both GitHub matrix jobs are green.

[junior-ricon]

Status: merge-ready. Freshened CI cleanup review; diff only adds the PR/push/workflow_dispatch test workflow using actions/checkout@v6 + jdx/mise-action@v4, local mise run test passed (20/20), git diff --check passed, and both Ubuntu/macOS checks are green.