Feature request: customizable thinking/spinner messages

## Feature Request

Allow users to customize the thinking/spinner messages (e.g., "Unravelling...", "Pondering...", "Thinking...") that appear while the model is generating a response.

## Proposed Config

A `thinking_messages` field in `config.json`, similar to the existing `launch_messages`:

```json
{
"thinking_messages": [
"Investigating for the Politburo...",
"Conducting revolutionary analysis...",
"The Party is deliberating...",
"Consulting the means of production..."
]
}
```

One message would be randomly selected each time the model starts thinking, just like `launch_messages` selects a random startup message.

## Use Case

We run a multi-agent IPC system ([Red Party](https://github.com/LauraTDN/Red-Party)) with a Soviet theme. We've Sovietized every user-facing message — instructions, hook deny messages, install scripts, terminal output — but the thinking spinner is the last holdout. Being able to customize these messages would complete the theming and also be useful for anyone who wants to personalize their CLI experience (team branding, humor, localization, etc.).

## Current Behavior

The thinking messages ("Unravelling...", "Pondering...", etc.) are hardcoded and not configurable via `config.json` or any other setting.

## Alternatives Considered

- `launch_messages` exists but only affects the startup banner, not the thinking spinner.
- No environment variable or CLI flag controls this behavior.