Persist runtime permission mode changes as session defaults

Problem

Issue #258 covered making /yolo persist across sessions through the default permission configuration. There is still a related gap for permission changes made at runtime inside an active TUI session.

When a user enables /yolo, /auto, or changes mode through /permission, the active session updates immediately, but a newly opened terminal starts from the existing configured default instead of inheriting the most recent permission mode chosen in the TUI.

Expected behavior

Runtime permission mode changes should update the persistent default permission mode in the user config, so new terminals and new sessions start with the same permission mode the user selected most recently.

Suggested scope

• Persist successful /yolo, /auto, and /permission mode changes to the config file as the default permission mode.
• Do not persist if applying the runtime permission mode fails.
• Report config persistence failures clearly while keeping the already-applied runtime session state.

Trace the /yolo, /auto, and /permission command handlers and the runtime permission application path, then review the persistent default behavior from issue #258. Done means successful mode changes update the user config, failed applications do not persist, and config-write failures are reported without changing the active session state.