fix: declare collection dependencies for incremental builds

Add eleventyImport.collections to templates that access collections
via Nunjucks loops (not pagination frontmatter). Without this,
Eleventy's --incremental mode skips these pages when content changes.

- index.njk: posts, feed (homepage recent posts widget)
- feed.njk: feed (RSS must update with new posts)
- feed-json.njk: feed (JSON feed must update with new posts)
- slashes.njk: pages (slash pages index)
- categories-index.njk: categories (categories index)

slashes.njk

@@ -3,6 +3,9 @@ layout: layouts/base.njk
 title: Slash Pages
 withSidebar: true
 permalink: /slashes/
+eleventyImport:
+  collections:
+    - pages
 ---
 <div class="h-feed">
   <h1 class="text-2xl sm:text-3xl font-bold text-surface-900 dark:text-surface-100 mb-2">Slash Pages</h1>