Codex security review found that migrating os.WriteFile(path, data, 0600)
to coreio.Local.Write() changed permissions from owner-only to world-readable
(0644). This is a security regression for encryption output, private keys,
and auth hashes.
WriteMode(path, content, mode) allows callers to specify permissions.
Write() remains the default (0644) for non-sensitive files.
Affected implementors updated: local.Medium, MockMedium, Node, datanode.Medium.
Co-Authored-By: Virgil <virgil@lethean.io>