Fix canonical metadata for public routes

[EmersonBraun]

What changed

• stop inheriting the homepage canonical across public routes
• add self-referencing canonicals for the homepage, ecosystem, community, evals, stack, learn, and showcase surfaces
• cover all generated learn and showcase routes with a canonical metadata regression test

Why

The root layout declared the homepage canonical globally. Next.js propagated it to child routes that did not override alternates.canonical, causing distinct public pages to advertise the homepage as their preferred URL.

Impact

Thirty-two affected public routes now emit their own canonical URL, while the homepage keeps the existing root canonical and localized alternates.

Validation

• npm exec -- vitest run apps/docs-next/tests/canonical-metadata.test.ts --coverage --coverage.include=apps/docs-next/lib/canonical-url.ts
• pnpm --filter @agentskit/docs-next lint
• pnpm docs:bridge:gate
• production Next.js build
• inspected 33 generated HTML files and verified each canonical exactly matches its route

[github-actions]

Lighthouse

URL	Performance	Accessibility	Best practices	SEO
http://127.0.0.1:3000/	66	92	96	100
http://127.0.0.1:3000/docs	67	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	92	100	96	100
http://127.0.0.1:3000/showcase	92	100	96	100

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