docs(hugo-theme): document cover images and template blocks

hugo-theme/SKILL.md

@@ -3,7 +3,8 @@ name: hugo-theme
 description: >-
   Build, customize, and debug advanced Hugo CMS themes — template
   architecture, asset pipeline (CSS/JS/image processing), shortcodes and
-  render hooks, page bundles, Hugo Modules, performance, SEO, and CI/CD.
+  render hooks, page bundles, cover images, Hugo Modules, performance,
+  SEO, and CI/CD.
   Use when working on a Hugo theme or site template layer.
 license: MIT
 compatibility: Works with any agent framework supporting the Agent Skills format.
@@ -25,7 +26,8 @@ Intermediate-to-advanced patterns for Hugo CMS theme development. Load the relev
 | **Template Architecture** | v0.120+ | You need to set up base templates with blocks, understand template lookup order (kind/layout/type/section), create partials, use partial decorators (v0.154+), or work with shortcode fundamentals | `references/template-architecture.md` |
 | **Asset Pipeline** | v0.161+ | You're integrating Tailwind CSS v4 (`css.TailwindCSS`) or v3 (PostCSS), using Hugo Pipes for SCSS/JS bundling, setting up fingerprinting and SRI, building responsive images with srcset, or processing page/global/remote resources | `references/asset-pipeline.md` |
 | **Shortcodes & Render Hooks** | v0.112+ | You need complex nested shortcodes, raw HTML shortcodes, markdown rendering inside shortcodes, custom render hooks for links/images/headings/code blocks, or language-specific code block rendering (Mermaid, etc.) | `references/shortcodes-and-hooks.md` |
-| **Content Organization & i18n** | v0.126+ | You're working with leaf vs branch bundles, headless bundles, custom taxonomies, content adapters (v0.126+, dynamic pages), section-specific layouts, archetypes, or internationalization (translation tables, multilingual) | `references/content-and-i18n.md` |
+| **Content Organization & i18n** | v0.126+ | You're working with leaf vs branch bundles, headless bundles, cover images, custom taxonomies, content adapters (v0.126+, dynamic pages), section-specific layouts, archetypes, or internationalization (translation tables, multilingual) | `references/content-and-i18n.md` |
+| **Cover Images** | v0.120+ | You need to add cover/hero images to articles, support both page bundle resources and frontmatter paths, generate responsive srcsets, or handle the no-cover case gracefully | `references/cover-images.md` |
 | **Modules & Performance** | v0.109+ | You're using Hugo Modules (init, import, vendor, workspace), building theme components with mount configuration, optimizing build speed with `partialCached`, configuring cache TTLs, or using configuration-driven theming (params, cascade) | `references/modules-and-performance.md` |
 | **Design, UX & Accessibility** | v0.120+ | You need typography systems, accessible color palettes, design tokens, semantic HTML landmarks, ARIA patterns, keyboard navigation, accessible forms, content-first layouts, responsive navigation, engagement patterns (reading progress, dark mode toggle, sharing), Core Web Vitals optimization, container queries, `:has()` selectors, or testing/QA automation (axe-core, Lighthouse CI, visual regression) | `references/design-accessibility.md` |
 | **SEO, Output Formats & CI/CD** | v0.120+ | You need JSON-LD structured data, Open Graph / Twitter Cards, custom output formats (JSON, AMP), sitemap customization, or CI/CD pipelines for themes (GitHub Actions, testing, deployment) | `references/seo-outputs-testing.md` |
@@ -88,3 +90,4 @@ ls public/ | head
 - **`resources.Get` looks in `assets/`, not `static/`.** Files in `static/` are copied verbatim and not processed by Hugo Pipes. Use `assets/` for any file that goes through Pipes.
 - **Content adapter templates MUST use `_content.gotmpl` naming.** Regular `.md` files in the same directory are ignored when a `_content.gotmpl` exists.
 - **Render hook templates go in `_markup/` subdirectories.** Not in `_default/` directly — they need `layouts/_default/_markup/render-link.html` or section-specific `layouts/<type>/_markup/`.
+- **`block` in partials conflicts with `define` in page templates.** `{{ block "title" . }}` inside a partial (e.g. `head.html`) uses the same Go template namespace as `{{ define "title" }}` in page templates (e.g. `single.html`). When both exist in the render tree, Hugo errors with `multiple definition of template "title"`. Fix: use direct page variables (`.Title`, `.Site.Title`) in partials instead of `block`. Reserve `block` exclusively for the `baseof.html` shell.

hugo-theme/references/template-architecture.md

@@ -164,3 +164,16 @@ layout: "wide"
 ---
 ```
 Renders `layouts/<type>/wide.html` instead of `layouts/<type>/single.html`.
+
+### Pitfall: `block` in partials conflicts with `define` in page templates
+
+`block` and `define` share the same Go template namespace. A `{{ block "title" . }}` in a partial (e.g. `head.html`) conflicts with a `{{ define "title" }}` in a page template, producing `"partials/head.html: template: multiple definition of template 'title'"`.
+
+**Fix:** Do not use `block` in partials. Use direct template expressions instead:
+
+```go-html-template
+{{- /* Good: partial without block */ -}}
+<meta property="og:title" content="{{ .Title }} | {{ .Site.Title }}">
+```
+
+Leave `block` only in `baseof.html` for child templates to fill via `define` at the page level.