fix: Upgrade Vitest 3→4 to resolve npm audit vulnerabilities

[dean0x]

Problem

npm audit reports vulnerabilities in the Vitest 3.x dependency chain. Vitest 4.x resolves these.

Scope

1. Upgrade vitest, @vitest/coverage-v8, @vitest/ui from ^3.2.4 to ^4.x
2. Review Vitest 4 migration guide for breaking changes
3. Update vitest.config.ts if config format changed (especially memoryLimit, worker pool, singleThread)
4. Run all test groups and verify they pass:
   • npm run test:core
   • npm run test:handlers
   • npm run test:repositories
   • npm run test:adapters
   • npm run test:implementations
   • npm run test:cli
   • npm run test:integration
   • npm run test:worker-handler
5. Verify npm audit is clean (0 vulnerabilities)
6. Verify npm run build succeeds

Risk

Semver major upgrade — config format, worker pool behavior, and API surface may change. The memoryLimit and singleThread options are critical for our memory management strategy.

Acceptance Criteria

• All test groups pass
• npm audit reports 0 vulnerabilities
• npm run build succeeds
• Memory management still effective (workers restart at threshold)

Dependencies

• Depends on NPM_TOKEN rotation and release.yml removal (need green CI to verify)