Every SaveAsync resets profiles.json, commands.json and active-profile.json to 0644 and replaces symlinked data files with regular ones #221

What's wrong

JsonKeybindingRepository.WriteFileUnlockedAsync (Keybinding/Services/JsonKeybindingRepository.cs, lines ~403–419 at 89f14b9) does this for each data file:

await File.WriteAllTextAsync(tempPath, contents);
File.Move(tempPath, path, overwrite: true);

The temp file is created fresh, so after the move:

• On Unix: each data file has the umask default mode (usually 0644), whatever mode it had before. A user who restricts the keybinding data to 0600 gets world-readable files back on the next save.
• When a data file is a symlink: the rename replaces the link with a regular file, and the link's target stops being updated.
• On Windows (not tested): MoveFileEx with replace leaves the temp file's inherited ACL in place of any explicit ACL the original had. File.Replace would keep it.

Reproduction (Linux, .NET 10)

KeybindingManager initialise → bind file.save to Ctrl+S → SaveAsync → chmod 0600 all three files → rebind → SaveAsync:

active-profile.json: 0600 -> OtherRead, GroupRead, UserWrite, UserRead
commands.json:       0600 -> OtherRead, GroupRead, UserWrite, UserRead
profiles.json:       0600 -> OtherRead, GroupRead, UserWrite, UserRead

Suggested fix

• When path exists on non-Windows, create the temp file with UnixCreateMode = File.GetUnixFileMode(path) (via FileStreamOptions), or call File.SetUnixFileMode on it before the move.
• Use File.Replace(tempPath, path, null) when the destination exists, which keeps the Windows ACL. Keep File.Move for the first write.
• Resolve a symlinked path to its final target and write the temp file next to that target.

Acceptance criteria:

• After a save over existing 0600 files, they are still 0600.
• A symlinked data file is still a symlink after a save, and its target holds the new content.
• Saves stay atomic: the target is never truncated in place.

Navigation's JsonFilePersistenceProvider has the same problem and is filed separately.