kg: add /concepts/ list-page layout (concept index) #686

Follow-up from PR #679 (Phase 3 Track 3-A, minor finding from final review).

Context

scripts/fetch-concepts.ts writes a _index.md to hugo/content/concepts/ so Hugo renders a /concepts/ index page. The Hugo concept-page template at hugo/layouts/concepts/single.html includes a breadcrumb <a href="/concepts/">Concepts</a> linking to that index.

But no hugo/layouts/concepts/list.html was added in PR #679. Hugo falls back to its default _default/list.html resolution — which on this project may render as an empty page or 404 depending on the default layout's expectations.

What this issue delivers

A dedicated hugo/layouts/concepts/list.html rendering a clean index of all published concepts, grouped or alphabetized as makes sense. Sibling to the existing single-page layout. Ideas for content:

• Page title: "Concepts"
• One-line intro explaining what concept pages are ("Topics extracted from tutorials, linking related learning paths")
• A simple grid or list of all published concepts, alphabetical by name
• Each item: concept name (linked to /concepts/<slug>/) and a 1-line description (truncated)

Take cues from hugo/layouts/_default/list.html and hugo/layouts/tutorials/list.html (if it exists) for the project's conventions.

Acceptance

• /concepts/ renders an index of all published concepts.
• Breadcrumb <a href="/concepts/">Concepts</a> on individual concept pages resolves to a non-empty page.
• Layout is responsive (mobile-friendly).
• Hugo build prints no warnings about missing layouts.

Refs

• PR feat(kg): Phase 3 Track 3-A — concept landing pages (#446) #679 (parent — left the index page as default-layout fallback)
• scripts/fetch-concepts.ts (writes _index.md)
• hugo/layouts/concepts/single.html (the breadcrumb link)