fix(utils): handle EXDEV in atomic_replace for cross-filesystem symlinks

[maxmilian]

Summary

atomic_replace (used by every atomic_json_write / atomic_yaml_write site) failed with OSError: [Errno 18] EXDEV when the target was a symlink whose real file lives on a different filesystem — e.g. a managed deployment that symlinks ~/.hermes/config.yaml / SOUL.md / auth.json to a git-tracked profile package on another mount. Every atomic write to such a target blew up.

Fixes #36653.

The bug

atomic_*_write stages the temp file in path.parent (next to the symlink, on filesystem A). atomic_replace then resolves the symlink to its real target (on filesystem B) and calls os.replace(tmp, real_path) — which can't rename across devices, so it raises EXDEV. The non-symlink path is unaffected (temp and target share a directory, hence a filesystem).

Changes

• utils.py: atomic_replace now catches EXDEV and delegates to a new _replace_across_devices helper, which re-stages the bytes on the target's own filesystem (copy + fsync + os.replace within that fs) so the swap is still a real atomic rename. The original cross-device temp is always cleaned up. Non-EXDEV OSErrors propagate unchanged.

Tests

• tests/test_atomic_replace_symlinks.py — 3 new regression tests (11 total pass):
  • cross-device symlink fallback preserves the symlink and writes the real file
  • atomic_json_write survives a cross-device symlink target
  • non-EXDEV OSError is re-raised and leaves the target untouched
• Existing 8 symlink tests + test_atomic_json_write.py / test_atomic_yaml_write.py (17) still pass.
• ruff check (PLW1514) clean; scripts/check-windows-footguns.py clean (binary mode + errno.EXDEV, cross-platform safe).

Scope

• Only atomic_replace changes; no caller signatures touched. Permission-restore (_restore_file_mode) is unaffected — it still targets the returned real path.
• EXDEV is simulated via mock (the issue's suggested approach) rather than a real second mount, so the test runs anywhere.
• EXDEV detection keys on the POSIX errno.EXDEV semantics; on Windows the prior behavior is preserved unchanged (the fallback simply may not trigger for cross-volume moves).

[maxmilian]

Closing — atomic_replace in utils.py on main already handles this: it resolves symlinks first and falls back to copy/fsync/unlink on EXDEV/EBUSY for cross-filesystem/bind-mount deployments. The gap this PR targeted is covered upstream.