Replace afero.TempFile (which uses os.CreateTemp with mode 0600)
with a custom createTempFile that uses OpenFile with mode 0666.
This lets the kernel apply the process umask, matching the default
behavior of os.Create. New files now get ~0644 (with standard
umask) instead of 0600.
Extract atomicWrite(ctx, path, mode, haveMode, reader) to share
the entire temp-file lifecycle between writeFile and editFile.
Mathias Fredriksson
f3b91b7f11