docs: add a hero banner to the top of the README (#133)

Closes #132

## What

Adds a rendered hero banner at the top of the README and stores it in a
new `assets/` directory.

## Why

The README opened with a text-only block. The banner puts the AI-agent
positioning and real command output above the fold, and gives the
project a reusable visual identity for blog posts and release
announcements. The same artwork is the cover image for an upcoming
dev.to article.

## Notes

- The image is committed at 2x (2000x840) and displayed with
`width="1000"`, so it downsamples crisply on high-DPI screens.
- The `# SlackCLI` H1 is dropped, because the banner carries the
wordmark in large type and the two stacked read as duplicated. The
project name is preserved in the image `alt` text, and no internal
anchor links pointed at that heading.
- No source or configuration files are touched, so the type-check and
test hooks skipped with nothing to check.