fix: handle EXDEV cross-device errors in atomic_replace

[OmarB97]

Summary

When config files (e.g. ~/.hermes/config.yaml) are symlinked to a different filesystem — common on WSL where dotfiles live under /mnt/c/ — os.replace() fails with EXDEV (errno 18: Invalid cross-device link).

This causes every hermes config set command to crash with:

OSError: [Errno 18] Invalid cross-device link: '/home/user/.hermes/.config_xxx.tmp' -> '/mnt/c/Users/.../dotfiles/hermes/config.yaml'

Fix

Catch EXDEV in atomic_replace and fall back to shutil.copy2 + os.unlink. copy2 preserves metadata (permissions, timestamps) so the result is equivalent. Non-EXDEV errors (EACCES, etc.) still propagate normally.

This affects all atomic write paths (atomic_yaml_write, atomic_json_write, atomic_roundtrip_yaml_update) since they all route through atomic_replace.

Test Plan

• 4 new regression tests (all passing):
  • Direct EXDEV fallback on plain file
  • EXDEV fallback preserving symlinks
  • Non-EXDEV errors are not swallowed
  • End-to-end atomic_yaml_write EXDEV recovery
• All 12 existing symlink tests still pass

Context

WSL users who symlink their dotfiles repo to ~/.hermes/config.yaml hitting a Windows mount path under /mnt/c/. The temp file is created in the WSL filesystem (~/.hermes/), but os.replace's underlying rename() syscall can't cross filesystem boundaries.

Fixes #34252 — credit to @ccwssy for the root-cause analysis of the cross-device rename failure.

[teknium1]

Thanks for the EXDEV investigation and regression coverage.

Automated hermes-sweeper review found this is already implemented on current main:

• utils.py:114-135 catches EXDEV (and EBUSY) and performs copy, metadata preservation, fsync, and temp-file cleanup while retaining the resolved symlink target.
• tests/test_atomic_replace_symlinks.py:245-349 already covers the mocked EXDEV path, symlink preservation, non-EXDEV propagation, and a real cross-filesystem case.
• This landed in bf8effad0 (fix(utils): copy fallback for atomic replace across devices, fix(utils): EXDEV/EBUSY copy fallback in atomic_replace (port from gemini-cli#21541) #43852), included in v2026.6.19.

The linked #34252/#34262 discussion concerns the same root cause. Closing as superseded by current main.