fix: handle cross-device config writes

[han-dreamer]

Summary

Fix config writes when renameSync(tmp, path) fails with EXDEV.

This addresses #61.

Changes

• Keep the existing atomic renameSync path for normal config writes.
• Add an EXDEV fallback that copies the temp config file to the final path and then removes the temp file.
• Preserve existing behavior for non-EXDEV errors by rethrowing them.
• Add tests for:
  • EXDEV fallback behavior.
  • Non-EXDEV error rethrowing.
  • Normal config write path.

Why

Issue #61 reports:

EXDEV: cross-device link not permitted, rename '...\config.json.tmp' -> '...\config.json'

[TumCucTom]

oh — I also opened a competing fix for #61 (PR #171) without realising yours was already in flight, the user moved fast and I didn't sweep the open list. closing #171 in favor of yours.

your design is the better one: exporting renameWithCrossDeviceFallback as a generic helper (not just for the config file) with the ops parameter for DI is much cleaner than what I did. the three tests covering happy/fallback/rethrow are the right minimum.

one small thing: the EXDEV-fallback and rethrow tests only check the sequence of mocked calls, not that the file actually gets written. an end-to-end test that injects a throwing renameWithCrossDeviceFallback into writeConfigFile and verifies the file content lands would close the loop on user-facing behavior. if a future refactor breaks the wiring between writeConfigFile and the helper, neither of the current tests would catch it.

style nit: (err as NodeJS.ErrnoException).code !== 'EXDEV' — a tiny isExdevError type guard would read cleaner than the cast.

approved. thanks for the better design.

[han-dreamer]

Thanks for the detailed review!

I pushed a follow-up commit that addresses both suggestions:

• Added isCrossDeviceError() to make the EXDEV check more readable.
• Added an end-to-end writeConfigFile test that injects an EXDEV rename failure, uses the real copy/unlink fallback, and asserts the final config file content.

Validation:

• npx bun test test/config/loader.test.ts test/commands/config/set.test.ts
• npx tsc --noEmit
• npx eslint src/config/loader.ts test/config/loader.test.ts