Add a short demo GIF and share-safe public demo output

[SuvenSeo]

Goal

Add a short README demo GIF or screen recording plus share-safe demo output generated from synthetic data.

Why

A reviewer or first-time user should understand the dashboard in less than 30 seconds without installing anything.

Acceptance Criteria

• README shows a short GIF or recording near the top.
• Demo uses only synthetic data from codex-usage-tracker demo.
• The demo does not include private local paths, prompts, responses, or real project names.
• Docs explain how to regenerate the demo artifact.