fix(server): stabilize LocalStorageProvider mtime unit test

[turbofoxwave]

Summary

• Fixes the flaky LocalStorageProvider unit test that failed CI when filesystem mtime landed 1ms ahead of Date.now().
• Splits remove vs mtime into separate cases and asserts a finite mtime.getTime() instead of wall-clock or toBeInstanceOf(Date) (Jest Date-realm false negative).

Context

Latest Test workflow on main failed with:

Expected: <= 1785077376681
Received:    1785077376682

in packages/server/src/providers/storage/local.test.ts.

Local verification

• npm test: 67 suites / 1049 tests passed (core, cursor, cli, server)
• npm run test:server: 16 suites / 232 tests passed

Broader unit-test review notes

Suite is healthy overall (config layers covered, tmpdir isolation, coverage gates). Non-blocking smells observed while reviewing:

• Private session Map mutation in simple.test.ts (prefer fake timers)
• Some path assertions couple to os.homedir()
• Mega suites: installer.test.ts, manifest.test.ts
• Occasional Jest open-handle warning after slow git-registry-client.test.ts

Test plan

• Review CI failure vs local suite
• npm test
• npm run test:server
• Confirm CI Test workflow green on this PR