docs(readme): video demo + restructure hero and prompt-to-production flow

[santoshkumarradha]

Summary

Replace the static hero image with an embedded video demo, simplify the heading, and restructure the top of the README so the install path is the first thing developers see.

What changed

• Hero image removed. Replaced with a GitHub user-attachment video preview (centered, with a one-line caption explaining what the demo shows).
• Title merged. # The AI Backend → # AgentField — The AI Backend (single em-dash heading instead of two stacked lines).
• "Build it with a prompt" leads. Curl install + the /agentfield skill spec are now the first content block after the hero, not the seventh section. This is the lowest-friction onboarding path and deserves top position.
• Code example reframed as "The DX you get". Same Python claims-processor example, now framed as evidence of DX quality with a clear one-line preamble. The "Prefer to scaffold by hand?" section is collapsed into a <details> block (Python/Go/TS/Docker variants all together).
• Hero asset updated (assets/github hero.png) to a cinematic 21:9 typographic version. Not referenced in the README right now — saved for future use (e.g., social card, future hero swap).

Why

From the analytics:

• 41% of README visitors are "Explorers" who skim, then leave — they need video, not paragraphs.
• 4% are "Installers" — they need the curl command above the fold, not buried below code.
• The video + curl-first structure serves both audiences without compromising either.

The static hero image was doing the same job as the video, but worse. Removing it saves vertical space and removes redundancy.

Test plan

• Render the README on GitHub and confirm the video player loads inline
• Verify the curl install command and /agentfield snippet are visible above the fold on a standard laptop screen
• Click each UTM-tagged link to verify they still resolve to the correct page
• Confirm the "Prefer to scaffold by hand?" details block expands cleanly