feat: scope the sidebar to the current category

Assisted-by: Delta:zai-glm-5.3

README.md

@@ -7,6 +7,7 @@ site generator.
 ## Layout
 
 - `site/` — website sources: `calepin.toml` + `.typ` pages
+- `site/themes/yggdrasil/` — local theme extending the bundled `calepin` theme
 - `public/` — generated site (gitignored)
 - `tools/` — pinned `calepin` and `typst` binaries (gitignored, downloaded on demand)
 - `scripts/` — helper scripts used by the Justfile and CI
@@ -59,6 +60,12 @@ Sidebar labels come from the page's `#metadata(...)` line, not from the config.
    top navigation bar. The `label` is required here because the index page's
    metadata title is `Overview`.
 
+Pages inside a category show only that category's sidebar section — the
+local theme (`site/themes/yggdrasil/partials/site-nav-sidebar.html`) filters
+the sidebar to the section containing the current page. Pages outside every
+category (landing page, `404`, standalone pages) fall back to the full
+sidebar.
+
 ## Deployment
 
 `just build` produces a fully static site in `public/`: HTML pages plus