docs(readme): add theme-aware banner

[johnnygreco]

Summary

Replace the README's single SVG banner with theme-aware PNG variants so the OpenShell wordmark remains legible in both GitHub light and dark modes.

Related Issue

No issue required: this is a localized README branding fix.

Changes

• Add light- and dark-mode OpenShell banner PNGs.
• Select the appropriate banner with GitHub's <picture> and prefers-color-scheme pattern.
• Limit the rendered banner width to 430 pixels.
• Follow GitHub's recommendation for light/dark mode images.

Before: current banner in dark mode

After

Testing

• mise run pre-commit passes
• Unit tests added/updated (not applicable; README and image assets only)
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[copy-pr-bot]

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[github-actions]

All contributors have signed the DCO ✍️ ✅
_{Posted by the DCO Assistant Lite bot.}

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-2619.docs.buildwithfern.com/openshell

[johnnygreco]

I have read the DCO document and I hereby sign the DCO.

[johnnygreco]

/ok to test e656db1