docs(feed): record the final cover image prompt on the special missings post

Replaces the placeholder one-liner with the prompt actually intended for the
cover: the pack of wolves on a snow plain at dusk with one animal standing
apart, plus the negative constraints, the output spec (./cover.jpeg at
1200x627, previewImg in front matter) and a fallback single-subject variant.

Two deliberate choices are recorded so they are not lost on a regenerate:

- No text, letters or numbers anywhere in the image. The subject is letters
  standing in for numbers, so a stray glyph undercuts the cover.
- No exact head count of 28. Generators cannot count, and a crowded pack reads
  worse than a dozen clear animals; the number belongs in the headline.

Links to the full prompt and variants on paste.4gl.io.

content/feed/sas-special-missings/index.md

@@ -90,5 +90,32 @@ The video shows the whole cycle - entering them, the rejections, submit, approve
 
 #sas #datacapture #mdm #dataquality
 
-Image prompt: a pack of 28 wolves fanning out across a snowy plain at dusk, one wolf standing apart to the left, wide cinematic composition, cool blue-grey palette with a single warm amber accent on the lone wolf, soft depth of field, no text and no logos, 16:9, editorial cover art for a technical blog post about the 28 SAS missing values. Save the generated image as ./cover.jpeg and set previewImg: './cover.jpeg' in the front matter above.
+Image prompt: Cinematic editorial cover illustration: a large pack of wolves, a
+dozen or more, spread wide and moving together across a vast snow plain at dusk,
+seen from a low wide angle. One wolf stands apart from the group on the left of
+frame, turned back toward the pack. The pack is rendered in cool blue-grey and
+silver; the lone wolf catches the only warm light in the scene, a single low
+amber sun. Overcast dusk sky, faint falling snow, long soft shadows, generous
+empty sky and snow to the upper third so the wide crop breathes. Painterly
+digital illustration, muted desaturated palette, soft depth of field with the
+distant wolves falling out of focus, no text, no letters, no numbers, no logos,
+no watermark.
+
+Constraints: no text, no letters, no numbers, no digits, no symbols, no
+captions, no logos, no watermark, no signature, no border, no frame, no collar,
+no harness, no humans, no buildings, and do not ask for an exact head count of
+28 - a crowded pack reads worse than a dozen clear animals, and the number
+belongs in the headline, not the artwork. The no-text rule matters more than
+usual here: the subject is letters standing in for numbers, so a stray glyph
+anywhere undercuts the cover.
+
+Output: save as ./cover.jpeg at 1.91:1 (1200x627, matching the other feed
+covers and doubling as the LinkedIn share card), and set
+previewImg: './cover.jpeg' in the front matter above. Do not also embed the
+image in the markdown body. Full prompt, variants and rationale:
+https://paste.4gl.io/?513711f5bde3610e#9LZCA9xzQHDy3CycXmsWU5AoVNtjBZmPotUHHtHTAerF
+
+Fallback variant if the pack composition comes back muddled: a single wolf
+standing alone on a snow plain at dusk, its shadow stretching toward a distant
+pack reduced to small silhouettes on the horizon.
 -->