Package identity persists the user's name and email across runs so the
identify screen can pre-fill the form. Stored as a plain JSON file in the
OS-native user-config directory — this is convenience data, not a secret.
Path returns the absolute path to the identity file under the OS user
config dir, creating the parent dir on demand. An error here is fatal —
callers can't persist without it.
Save writes the identity to disk. Non-atomic — good enough for a file this
small; a truncated write still parses or silently returns zero values on
next load.