[FEATURE] Add non-interactive /config for temporary mid-session setting changes

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The only way to change settings (model, theme, etc.) mid-session is the interactive /config dialog. There's no text command for temporary changes without arrow-key navigation which is slow. Editing settings.json persists as new defaults, which is wrong when you only want a temporary switch (e.g., opus for one task, then back to sonnet).

Proposed Solution

Support non-interactive syntax:

/config model opus
/config theme dark

Same temporary session-only change the dialog makes, just without the dialog.

Alternative Solutions

• --model flag: only works at session start
• ANTHROPIC_MODEL env var: requires new session
• settings.json edit: changes persistent default
• Interactive dialog: works but unnecessary when you know the setting and value

Priority

Low - Nice to have

Feature Category

Configuration and settings

Use Case Example

User wants to switch to opus for a complex task, then back to sonnet afterward. Currently must use the /config dialog (arrow keys) twice. A command like "/config model opus" would accomplish the same thing faster without navigation overhead.

Additional Context

_No response_