Add polished Folderbase README banner

[jerelvelarde]

Summary

• add a polished 1600×640 light-theme Direction C banner to the Core README
• position Folderbase as the open folder database for AI agents with a clear folder → .folderbase/ → agent system diagram
• keep the banner reproducible from committed HTML/CSS/SVG source and a deterministic Playwright capture script
• add compact Apache 2.0, Rust, and CI badges while preserving the technical README body

Test plan

• render twice and verify identical SHA-256 digests
• verify RGB PNG at exactly 1600×640
• visually inspect the original-resolution capture
• cargo fmt --all -- --check
• cargo test --workspace --all-features --locked
• git diff --check