test: migrate from tap and c8 to node test runner - #150

Replace the tap and c8 development dependencies with Node.js built-in testing and coverage functionality.

This migrates the suite to:

• node:test with describe() and test()
• Native TestContext assertions
• Built-in test coverage and LCOV reporting
• Explicit promise handling for subprocess and event-driven tests

This also lets core-validate-commit, as a Node.js project, dogfood Node.js’s own test runner and coverage tooling.

The CLI’s --tap output option remains unchanged.

Assisted-by: codex:gpt-5.6-sol