docs: add README banner (rendered with B3's marketing CLI)

[Thanukamax]

What

Adds an on-brand hero banner to the top of the README — b3-cf-stack was the
only one of our OSS repos without one (the GitHub card was the auto-generated
default). This is the "fucked logos / update that image" item.

How it was made

Rendered with our own marketing CLI (BitByBit-B3/marketing) — the
hero-card template, og format (1200×630). The JSON brief is in that repo as
examples/create-b3-cf-app.json (separate PR). So it's fully on-brand and
reproducible: same input → byte-identical PNG.

• 1200×630, so it also works as the repo's social preview (Settings →
  General → Social preview — needs a maintainer with repo settings access to
  upload assets/banner.png there).

Note

Getting a clean og render needed a one-line fix in the marketing CLI's
hero-card og template (it wasn't passing the compact flag to the headline,
so large headlines overflowed into the subtitle). That fix is a separate PR on
the marketing repo. This PR just ships the finished PNG, so it's independent.

Draft for your review — swap copy/wording if you want a different headline than
"One command."

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