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."

Summary by CodeRabbit

• Documentation
  • Added a centered banner image with descriptive alt text to the top of the README.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: da98521f-dae3-4183-8fa3-2a824e6a875a

📥 Commits

Reviewing files that changed from the base of the PR and between 7071665 and 137dad6.

⛔ Files ignored due to path filters (1)

• assets/banner.png is excluded by !**/*.png

📒 Files selected for processing (1)

• README.md

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Walkthrough

README.md now begins with a centered banner image and descriptive attributes above the existing heading.

Changes

README presentation

Layer / File(s)	Summary
Add README banner README.md	Adds a centered banner image with descriptive alt text and a width attribute at the start of the README.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: prdai

Poem

A banner rises, centered and bright,
Welcoming readers with polished light.
Above the heading, it takes its place,
Adding a touch of visual grace.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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