Feature: Persist approval policy selection to config.toml

Problem

Currently, when a user changes the approval policy via the TUI (e.g., selecting 'Full Auto', 'Read-Only', or 'Untrusted' from the Approvals menu), the change is only effective for the current session. Restarting Codex causes it to revert to the default setting.

Proposed Solution

We should persist these changes to config.toml so that the user's preference is remembered across sessions.

Implementation Details

This would likely involve:

1. Adding a set_approval_policy method to ConfigEditsBuilder.
2. Dispatching a persistence event (e.g., PersistAskForApprovalPolicy) when the user selects a preset in the TUI.
3. Updating the App event handler to write the new policy to disk.

(Note: Implementation logic was explored in closed PR #8474)