Ctrl+T thinking toggle provides no screen reader feedback for state changes

### Describe the feature or problem you'd like to solve

When pressing Ctrl+T to toggle the reasoning/thinking display on or off, there is no textual or audible feedback indicating the current state of the toggle. Screen reader users have no way to know whether thinking mode is enabled or disabled after pressing the shortcut.

The status bar does not change to reflect the thinking state -- it shows the same content (`/ commands . ? help` and the model name) regardless of whether thinking is on or off. The only way to discover the current state is to submit a prompt and observe whether reasoning text appears in the response.

This was tested on Windows with NVDA and Copilot CLI v1.0.36, with the `screenReader` setting enabled in `settings.json`. Pressing Ctrl+T produces no speech output and no visible text change that the screen reader can detect.

### Proposed solution

When Ctrl+T is pressed, output a brief status message to the terminal indicating the new state, for example:

- "Thinking: enabled"
- "Thinking: disabled"

This would allow screen readers to announce the change and give all users confirmation that the toggle took effect. The `beep` setting could also optionally produce distinct tones for each state.