docs(readme): add cicd-squabbler banner image (#20)

Adds the README banner image (`cicd-squabbler.png`) and embeds it at the
top of `README.adoc` at 100% width.

The work was already done on the `banner/cicd-squabbler` branch (commit
`adb6f23`, signed, co-authored by Mistral Vibe) and pushed to origin —
but **no PR was ever opened**, so it never reached `main`. This opens
that PR.

Closes #17.

Note: `#17` claimed the branch was empty. That was a mis-diagnosis on my
part — the branch has always contained the image.

Author: hyperpolymath

README.adoc

@@ -4,6 +4,8 @@
 :toc: preamble
 :icons: font
 
+image::cicd-squabbler.png[CICD Squabbler Banner,width="100%",link=""]
+
 image:https://img.shields.io/badge/License-MPL_2.0-blue.svg[code: MPL-2.0]
 image:https://img.shields.io/badge/docs-CC--BY--SA--4.0-green.svg[docs: CC-BY-SA-4.0]
 image:https://img.shields.io/badge/invariant-SPARK--proved-success.svg[squabble≠bypass: proved]