fix(coding-agent): clean up Windows update and test workers

[bokjk]

Summary

• keep the detached OMO update worker's log handle alive until asynchronous close completes, and await worker startup before the foreground command returns
• group per-worker SENPI_CODING_AGENT_DIR quarantines under a per-run Vitest root and remove that root during global teardown
• retain standalone setup cleanup on natural process exit and add focused regressions for both lifecycle paths

This addresses the Windows UV_HANDLE_CLOSING teardown race and prevents normal repeated test runs from accumulating senpi-vitest-* directories.

Verification

• npm test -- test/omo-local-update-worker.test.ts test/omo-local-update-dispatch.test.ts test/setup-cleanup.test.ts — 3 files, 8 tests passed after rebasing onto current upstream main
• npm run check — passed after rebase
• npm run build — passed
• node .agents/skills/senpi-qa/scripts/lib/common.mjs --self-check — 9/9 passed
• node .agents/skills/senpi-qa/scripts/cli-smoke.mjs --self-test — 8/8 passed
• isolated-TEMP run left zero senpi-* entries after Vitest teardown

Full-suite note

packages/coding-agent full Vitest was also run after a workspace build. The changed tests passed, but the existing Windows suite remained non-green: 755 files passed / 68 failed / 9 skipped; 6,665 tests passed / 163 failed / 53 skipped, plus one unhandled error. Failures were in unrelated existing Windows-sensitive areas (for example POSIX permission assertions, install-method fixtures, external-editor shell quoting, and snapshot differences).

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Summary by cubic

Fixes a Windows teardown race in the OMO update worker and stops vitest runs from leaving behind senpi-vitest-* temp directories.

• Bug Fixes
  • Made defaultSpawnWorker async and kept the detached worker’s log handle open until the asynchronous close completes; the dispatcher now awaits spawnWorker.
  • Grouped per-worker SENPI_CODING_AGENT_DIR under a per-run SENPI_VITEST_QUARANTINE_ROOT created via globalSetup and removed on global teardown; kept a process-exit cleanup fallback for standalone setup.
  • Added regression tests for log-handle ordering and quarantine cleanup.

^{Written for commit 08cd3b7. Summary will update on new commits.}

[bokjk]