feat: add soft-delete filter and content-warning support

Filter posts with `deleted: true` from all collections so soft-deleted
posts no longer appear on the blog. Add content-warning support: on
listing pages, CW posts show a warning label instead of content; on
single post pages, content is wrapped in a collapsible <details>.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

eleventy.config.js

@@ -1028,7 +1028,7 @@ export default function (eleventyConfig) {
   });
 
   // Helper: exclude drafts from collections
-  const isPublished = (item) => !item.data.draft;
+  const isPublished = (item) => !item.data.draft && !item.data.deleted;
 
   // Helper: exclude unlisted/private visibility from public listing surfaces
   const isListed = (item) => {