fix(tests): stabilize local Windows test runs by handling EPERM on temp dir cleanup

[kok-o]

Problem Statement

While the automated GitHub Actions CI runner on \windows-latest\ runs the entire test suite cleanly, developers running
pm test\ locally on Windows occasionally encounter \EPERM: operation not permitted\ errors during teardown of temporary project folders.

This happens because background file indexing, antivirus scanners, or lingering file descriptors temporarily lock directories before \s.rmSync(dir, { recursive: true })\ completes.

Objective

Make the local test runner completely resilient to transient Windows file locks so that
pm test\ runs 100% green on local developer machines without manual retries.

Scope of Work

1. Safe Teardown Helper:
   • Implement a shared test helper \safeRemoveDir(dir)\ that wraps directory cleanup with exponential retry and backoff (e.g. \s.rmSync(dir, { recursive: true, force: true, maxRetries: 5, retryDelay: 100 })).
2. File Handle Auditing:
   • Verify that tests involving \SafeWriter, \JournaledTransaction, and git worktrees explicitly close file descriptors before invoking assertions or teardowns.
3. Npm Cache / Sandbox Isolation:
   • Ensure local temporary npm cache directories used during \install.test.js\ or \update.test.js\ avoid conflicting locks with the global user .npm\ cache.

Acceptance Criteria

• All 350+ unit and integration tests run to completion on local Windows environments without \EPERM\ failures.
• No flaky test teardown crashes in CI or local developer machines.