Adopt Vitest for TypeScript tests and run them in pull request CI

[ho991217]

Context

yarn test currently builds the package and runs node:test files against dist.
The existing tests work, but there is no pull request workflow that runs them.

As more CODEOWNERS and CLI regressions are covered, testing TypeScript source directly would make the suite easier to develop.

Proposed work

• Add Vitest and its required Vite dependency.
• Migrate the existing CODEOWNERS tests to Vitest without losing their assertions.
• Make yarn test run the suite once and return a failing exit code when a test fails.
• Add a pull request workflow that installs dependencies with yarn install --immutable and runs yarn test, yarn typecheck, and yarn build.

Acceptance criteria

• yarn test passes from a clean checkout without prebuilt dist files.
• The existing CODEOWNERS regression test, and the additional cases in PR fix: group CODEOWNERS by directory and resolve paths from root #16 once merged, run under Vitest.
• A failing test produces a failed check on a pull request.
• The package build is still verified separately, since source-level tests no longer exercise the built output.