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.

Written by the indexing model from the issue text.