[FEATURE] Persist vim mode across sessions via settings.json

Feature request

/vim enables vim keybindings for the current session, but there's no way to persist this across sessions. The setting doesn't survive restart — you have to run /vim every time.

Expected behavior

A settings.json field (e.g. "vimMode": true) that enables vim mode by default for all sessions.

Current workaround

Manually running /vim at the start of each session.

Context

/config shows a vim mode toggle but it only applies to the current session. The settings.json schema doesn't include a vim mode field, so there's no file-based way to persist it.