fix: avoid duplicate persist error output (#459)

This keeps the I/O errors in `Error::source()` and limits `PersistError` and `PathPersistError` display text to persistence-specific context. That prevents chain-aware formatters from rendering the same I/O errors twice.

This follows the standard library guidance for wrapped errors (see https://doc.rust-lang.org/std/error/trait.Error.html#error-source).