Thumbnails: accept thumb.<ext> or *_thumb.<ext> child notes, auto-hide them from menu (shareHiddenFromTree), keep first-image fallback

AGENTS.md

@@ -51,10 +51,12 @@ CSS file (Gruvbox, light + dark, theme toggle without reload).
 - **Dates**: all visible dates (post cards, garden table) are rendered by the generator's
   `dateText()` helper as `Fri 2026-08-28` (English weekday + ISO date); `blog_data.date`
   stays ISO for sorting, `blog_data.dateText` is added for display. JSON-LD keeps ISO dates.
-- **Thumbnails** (`thumbSrc()`): prefer a child note of the article titled `*_thumb.*`
-  (e.g. `ngc281_thumb.jpg`) → `api/notes/<id>/download`; otherwise use the first `<img>`
-  from the note content. Layout: image left, summary right (`.blog-post-inner`),
-  `max-height:256px`, lazy-loaded. **There is no server-side resizing** – see "Limitations".
+- **Thumbnails** (`thumbSrc()`): prefer a child note of the article titled `thumb.<ext>` **or**
+  `*_thumb.<ext>` (e.g. `thumb.png`, `ngc281_thumb.jpg`) → `api/notes/<id>/download`; otherwise
+  use the first `<img>` from the note content. Layout: image left, summary right (`.blog-post-inner`),
+  `max-height:256px`, lazy-loaded. The generator auto-sets `shareHiddenFromTree` on matching
+  thumb child notes so they never appear in the blog menu. **There is no server-side resizing** –
+  see "Limitations".
 - Regenerates `feed.xml`, `sitemap.xml`, `state_map` (garden states) and `blog_data`
   (search index: id/title/url/summary/tags/category). Sets `shareDescription` from `summary`.