Add generated README hero art (banner, terminal demo, architecture diagram)

[BenMcGit]

What

Rendered README: https://github.com/zapier/sdk/blob/feat/readme-hero-art/README.md — the point of this PR is how it looks.

• Banner: pixel-art key on a keyring of app-colored charms — "Zapier handles the keys" — with the ZAPIER SDK wordmark
• Terminal demo: real captured CLI output replayed as an animated SVG — action-key discovery via list-actions, then run-action finding this very repo
• Architecture diagram: callers → @zapier/zapier-sdk → Zapier platform (OAuth grants) → the app catalog, with sdk.fetch as the escape hatch

How it's built

• All three SVGs are derived artifacts: committed generators under .github/scripts/readme-art/ (plain Node .mjs, matching the existing scripts), deterministic to the byte — seeded PRNG only, no timestamps
• npm run generate:readme-art regenerates; generate:readme-art:check byte-compares and now gates every PR in validate.yml (no credentials needed, forks included)
• The demo transcript is captured output, never typeset — provenance (date, commit, pinned CLI version, what varies between captures) documented in transcript.mjs; the connection id is masked, and stderr update-notifier noise omitted
• SMIL animation only (runs inside <img> on GitHub), self-contained dark card so it reads well in both page themes, resting state is the full transcript

Notes for review

• .github/scripts/README.md and AGENTS.md updated to describe the new check
• No behavior change to the corpus, skill, or CI audit

🤖 Generated with Claude Code