Add unit tests for core domain logic

Configure a test framework and add unit tests for important domain logic such as status transitions, reminder rules, and validation. Ensure tests run locally and in CI.

First, check if a test framework like Jest or Mocha is already configured in package.json. Look for existing test files to understand the project's testing patterns. The core domain logic likely resides in files handling status transitions, reminder rules, and validation. Write unit tests for these functions, ensuring they run locally via npm test and are integrated into the CI pipeline (check .github/workflows or similar CI config).

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