fix(check): honor Vitest worker configuration

[PLASMA-FR]

Summary

AGENT_DEVICE_VITEST_MAX_WORKERS configures Vitest, but check:affected appended --maxWorkers=4, overriding it. Delegate worker sizing to vitest.config.ts so its default, validated local override, CPU clamp, and CI policy apply. This completes the propagation boundary introduced in #1964.

Also address the maintainer's Coverage request by freezing the default readiness-budget test's clock. Its exact 120,000 ms assertion was racing real time across async mock calls. The test now uses the same clock discipline as its neighboring deadline cases.

Four files, +24/−29; the review follow-up adds only the readiness test fix.

Validation

Tested commit: 704b15b87e47b72765bd18bc5aafb269abdf0d53 (Node 24.13.1, pnpm 11.17.0, Linux).

• Current-main real-clock probe observed 119,998 ms. Controlled clock perturbation failed before the fix and passed after it.
• Readiness and hermetic worker-policy suites: 14 passed.
• pnpm check:affected:test: 65 passed.
• AGENT_DEVICE_VITEST_MAX_WORKERS=1 pnpm check:affected --run: all runnable gates passed, including build, lint, types, package, integration, and compatibility checks.
• Full local coverage: 10,033 passed; two failures reproduced on unchanged main (umask 0077 masks file mode; fabricated PID 101 is live here). Readiness: eight passed under instrumentation.
• Authoritative upstream Coverage and Integration Tests: pending on this head.

[PLASMA-FR]

Coverage completed 10,034 tests and failed only the unchanged packages/platform-apple/src/readiness/runtime.test.ts default-budget assertion: it observed 119,999 ms instead of 120,000 ms. This PR changes only scripts/check-affected/{checks,model.test,run.test}.ts, and the corresponding local exact-head affected gate passed. As an external contributor I cannot rerun the upstream Actions job (GitHub returns 403); could a maintainer please rerun the failed Coverage job? Failed job: https://github.com/callstack/agent-device/actions/runs/34449654022/job/102782254533

[thymikee]

No code findings at bce5469. Removing the command-line override correctly leaves worker sizing with Vitest configuration, and the regression assertions cover that boundary. The Coverage failure looks unrelated: it runs Vitest directly, not check:affected, and failed an unchanged readiness assertion by one millisecond. Ready for human review; a green Coverage rerun is still needed before merge.

[thymikee]

Please fix coverage