test: add fast Vitest unit suite

[Meir017]

Summary

• Add Vitest as the fast unit-test runner and make npm test run only tests/unit.
• Move pure parser/security/clipboard specs from tests/e2e into tests/unit and convert them from Playwright test APIs to Vitest.
• Split filesystem/git/backlog-writer coverage into tests/integration with a separate npm run test:integration command.
• Run unit and integration tests before the packaged Electron e2e workflow and document when to use each suite.

Validation

• npm test
• npm run test:integration
• npm run test:e2e -- --list