fix(docs): move banner into docs/ to restore root allowlist (#21)

**`main` is currently red on Estate Rules.**

#20 merged at 20:56:28Z and landed `cicd-squabbler.png` at the
repository root. `scripts/check-root-shape.sh` rejects it:

```
FAIL: 1 root entries are not on the allowlist:
  - cicd-squabbler.png
```

The fix existed on the PR branch (`8ac6d5f`) but was pushed ~seconds
after the merge, so it missed the train. This re-applies it against
current `main`.

**Change:** `cicd-squabbler.png` → `docs/cicd-squabbler.png`, README
reference updated to match.

**Why `docs/` rather than allow-listing the root entry:** the
allowlist's own doctrine admits root entries only where the tooling
requires them ("must be moved into the appropriate subdirectory, or
added here with a justification comment"). An AsciiDoc banner has no
such requirement, and `docs/` is already allow-listed. This keeps the
estate rule intact instead of widening it for a cosmetic asset.

Verified locally: `PASS: root matches allowlist (47 entries, 52
permitted)`.

Author: hyperpolymath

README.adoc

@@ -4,7 +4,7 @@
 :toc: preamble
 :icons: font
 
-image::cicd-squabbler.png[CICD Squabbler Banner,width="100%",link=""]
+image::docs/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]