Allow disabling Rubber Duck for a session

### Problem

I want the option to turn off Rubber Duck for an individual session. In some sessions I want to conserve tokens or move faster without an additional critique pass.

I can ask for this behavior in the prompt, but that is less discoverable and reliable than a dedicated control.

### Requested behavior

Add a per-session toggle or similar setting that disables Rubber Duck for that session. The setting should be easy to change without requiring custom prompt instructions.

### Use cases

- Reduce token usage when an additional review pass is not needed.
- Get to implementation faster for straightforward or time-sensitive work.
- Choose when the extra critique step is valuable on a session-by-session basis.

The issue names no files or tests; begin by locating the session controls and Rubber Duck flow in the desktop app. Determine where a per-session setting can be surfaced, then verify that changing it affects only the current session and skips the extra critique pass while leaving other sessions unchanged.

索引モデルが issue の本文から書いたものです。