Add A Hallmark Grinchmas to the books content collection

Marked draft so it doesn't render live yet -- this site has no
books layout/section wired up. For now this is just the canonical,
git-reviewable source for the book's title/author/cover/logline,
read by reground-tools/arc-campaign.sh --from-file to populate the
bookshop giveaway campaign, instead of typing that metadata inline
on the command line.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

content/books/grinchmas.md

@@ -0,0 +1,7 @@
+---
+title: "A Hallmark Grinchmas"
+author: "Marisol T. S. Kuhn"
+cover_image: "https://downloads.thistleandmoth.org/grinchmas/grinchmas-cover.png"
+description: "California native Bart Marley unexpectedly finds himself in the, to its mind perfect, midwestern town of Hallmark. One child refuses to give up on the stranger but is her innocence the Christmas miracle Bart needs?"
+draft: true
+---