Move config document helpers into their own module

[jif-oai]

Why

core/src/config/edit.rs owns the config edit state machine, but it also carried the TOML document helper code inline as a nested module. Moving those helpers into their own file keeps the edit orchestration easier to scan without changing the config persistence behavior.

What changed

• Moved the existing document_helpers module from core/src/config/edit.rs into core/src/config/edit/document_helpers.rs.
• Added mod document_helpers; so the existing pub(super) helper API remains available to the rest of config::edit.

Testing

Not run; this is a refactor-only module extraction with no intended behavior change.