[codex] Rewrite README gallery and quickstart

[fortunexbt]

What changed

The README gallery and quickstart have been rewritten around the actual ckitty experience. The banner and terminal demo now use stable raw GitHub URLs pinned to the commit that contains the current assets, so GitHub renders the exact files instead of falling back to stale relative paths from main.

The new README leads with the cat, gives a copy-paste install command, keeps the everyday commands small, documents the interactive controls and resize behavior, and moves deterministic rendering and contributor details lower down. It also removes a stale make run example that was not a real Make target.

This PR includes the current resize-safe TUI, aligned banner, slower real-frame GIF, and resize regression coverage from the preceding refinement. Merging the PR will put the README and the assets it displays on the default branch together.

Validation

• make check
• public banner/GIF URLs resolve with HTTP 200
• public asset SHA-256 hashes match the local files
• git diff --check