Add GitHub Actions workflow to run tests

[pyramation]

Summary

Adds a GitHub Actions workflow that runs all tests on push to main and on pull requests targeting main. The workflow:

• Uses pnpm v10 for package management
• Runs on Node.js 20.x
• Executes pnpm test which runs tests across all 3 packages (just-bash, zen-test, zen-bash)

Review & Testing Checklist for Human

• Verify this PR's CI check passes (the workflow should run on this PR itself)
• Confirm pnpm version 10 matches the project's packageManager field in root package.json

Notes

The tests require NODE_OPTIONS='--experimental-vm-modules' for ESM support, which is already configured in each package's test script.

Link to Devin run: https://app.devin.ai/sessions/e5a59ab234c5481e817dad43a74cfc6e
Requested by: Dan Lynch (@pyramation)

[devin-ai-integration]

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