[Feature Request] Add setting to disable automatic selection attachment in terminal mode

**Bug Description**
In VS Code terminal mode (claudeCode.useTerminal: true), text I select in the editor is attached to every prompt. claudeCode.attachOpenFile: false only stops the open-file notice, not the selection. Please add a setting to turn off selection sharing without disconnecting the IDE integration entirely.

**Environment Info**
- Platform: win32
- Terminal: xterm-256color
- Version: 2.1.274
- Feedback ID: e6c17f4e-9104-4fa8-a614-3c041ca274b8

**Errors**
```json
[]
```

Locate the existing claudeCode.attachOpenFile setting and the terminal-mode path that attaches selected editor text to prompts. Add a separate setting for selection sharing while preserving the IDE integration, then verify that disabling it stops selection attachments without affecting other integration behavior.

Written by the indexing model from the issue text.