Replace terminal demo SVG with custom artwork

[m3lixir]

Context

The current assets/terminal-demo.svg is a placeholder terminal-style preview for the README quickstart. It works functionally, but the visual direction should be replaced with a custom asset.

Goal

Create a new README demo image that better matches the project's style and presentation.

Acceptance criteria

• Replace or remove assets/terminal-demo.svg.
• Keep the README quickstart visually clear.
• Avoid clipped output or cramped terminal text.
• Make sure the asset renders correctly on GitHub.
• Keep the quickstart wording clear that the GPTBot/1.0 entry is a local fixture, not a real crawler visit.

Notes

This can wait until there is time to make a custom visual direction.