Add demo screenshots or GIFs to the README

[pouryawJs]

Summary

The README would be more useful with a small demo section showing the CLI and GitHub Action output.

Why this matters

Maintainer Agent Kit is easier to understand when users can see what happens during a dry-run triage or release notes generation.

Suggested demo assets

• CLI output for validate
• CLI output for triage
• CLI output for release-notes
• GitHub Action log screenshot
• Example issue receiving labels

Acceptance criteria

• Add at least one CLI screenshot or terminal output block
• Add one GitHub Action usage screenshot or log example
• Add a Demo section to README
• Keep images lightweight and readable