Fix raster icon fallbacks for site discovery

[EmersonBraun]

Summary

• add ICO and PNG favicon fallbacks alongside the existing SVG assets
• add a 180x180 PNG Apple touch icon to the documentation and landing sites
• expose the raster logo in Organization structured data
• document and test the public icon contract

Why

External directories and image pipelines commonly cannot rasterize SVG-only favicons. Infrabase surfaced this issue after its logo fetch failed. These fallbacks let crawlers and listing services discover a usable AgentsKit mark without relying on the GitHub avatar.

Validation

• pre-push quality gates: 42/42 tasks passed, including production builds for docs-next and landing
• pnpm vitest run apps/docs-next/tests/site-icons.test.ts apps/docs-next/tests/canonical-metadata.test.ts (8 tests passed)
• git diff --check
• verified ICO/PNG formats and 16x16, 32x32, and 180x180 dimensions

[github-actions]

Lighthouse

URL	Performance	Accessibility	Best practices	SEO
http://127.0.0.1:3000/	43	92	96	100
http://127.0.0.1:3000/docs	64	100	96	100
http://127.0.0.1:3000/docs/get-started/getting-started/quickstart	68	96	96	100
http://127.0.0.1:3000/stack	94	100	96	100
http://127.0.0.1:3000/showcase	92	100	96	100

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