Config files hold the auth secret and tokens, so the default 0o644 lets
every user on the system read them. Config.save, the ctl config written
by the controller bootstrap, ctl host create-config and the config
migration in create now pass mode 0o600.
safeOutputFile also applies the mode to the temporary file it writes,
since writeFile only honours mode on creation and a leftover temporary
from a failed write would otherwise keep its old mode.
Closes#1005
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
