feat: add marketing website and adopt cooperative-model terminology

Summary

• Add Eleventy-based marketing website under website/ with home, blog index, and first post (“Hello, Fabrik”). Vanilla CSS, Nunjucks templates, OG image generator script, RSS feed.
• Reframe Fabrik as a “polyglot automation kernel for humans and agents” across README, VitePress docs landing, What is Fabrik, and homepage hero.
• Rename “adopted” / “Cargo Escape Hatch” to the canonical “cooperative model” / “cooperative path” in AGENTS.md and docs/targets/rust.md. Code identifiers (cargo.binary, CargoBinaryTarget) unchanged.
• Align blog index header and post header with the navbar/footer container by removing centered max-widths; prose body inside posts stays narrow for readability.

Testing

• cd website && aube run build (Eleventy build succeeds, writes 4 pages and copies 22 assets)
• Not run: VitePress docs build (no changes that affect VitePress rendering beyond prose)

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