Move Grinchmas's landing page from bookshop into Hugo

Same fix as michaelferrara-site: bookshop's GET /arc/{code} renders a
generic, brand-agnostic template ("Get your free copy of A Hallmark
Grinchmas") instead of real editorial copy. bookshop already supports
same-origin form POSTs to its path-mounted /shop/arc/{code} — no reason
for it to also own the landing page.

/books/grinchmas/ is a plain Hugo page (content/books/grinchmas.md, no
longer draft-only) with a small JS form (layouts/partials/arc-redeem-
form.html, mirroring course-enroll-form.html) that POSTs form-encoded
to bookshop_base_url + "/arc/grinchmas". Retires content/gift,
content/gift-thanks, and content/free-book — all unfinished TODO stubs
that were live on the site with placeholder copy, never completed.

Copy note: no finished pitch copy existed anywhere for this campaign
(unlike Neon Tan) — this page currently uses the book's back-cover
blurb as placeholder pitch text per your instruction; refine whenever.

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

content/free-book/_index.md

@@ -1,11 +0,0 @@
----
-title: "TODO: Free Book Title"
-description: "TODO: one- to two-sentence hook for the free book being given away."
----
-TODO: free-book page copy goes here.
-
-This page is a stub — the link below won't resolve until the real ARC
-campaign is created (see reground-tools' `arc-campaign.sh`) and this copy
-is replaced with the real title/code.
-
-[Get your free ebook](https://thistleandmoth.org/shop/arc/TODO-real-code)

content/gift-thanks/index.md

@@ -1,14 +0,0 @@
----
-title: "Thanks! Here's your story."
-description: "TODO: one-line description once the real reader-magnet file is ready."
----
-
-TODO: this is a stub. Once the real file is uploaded to the
-`thistleandmoth-downloads` bucket, link it here the same way
-`content/gift/index.md`'s `download_url` does.
-
-[Download the story](https://downloads.thistleandmoth.org/reader-magnet/story.epub)
-
-Talk soon,
-
-Thistle & Moth

content/gift/index.md

@@ -1,38 +0,0 @@
----
-title: "TODO: Reader Magnet Title"
-description: "TODO: one- to two-sentence hook for the free story/piece being given away."
-type: "courses"
-eec_slug: "reader-magnet"
-
-# Optional italic line under the title, for handling an objection up front.
-hero_subhead: "TODO: this story, occasional emails when something new comes out, and other things you want in your inbox."
-
-# Label on the signup form's submit button.
-cta_text: "Send it to me"
-
-# Cover image, hosted in the public downloads bucket alongside the real
-# file. STUB — thistleandmoth-downloads is currently empty; these paths
-# won't resolve until the real cover/file are uploaded (see the tofu
-# linode_object_storage_bucket.thistleandmoth_downloads resource).
-cover_image: "https://downloads.thistleandmoth.org/reader-magnet/cover.jpg"
-
-# Optional: a landscape (1200x630) variant of the cover for social-share
-# previews (og:image/Twitter card). Falls back to cover_image if unset --
-# see layouts/_default/baseof.html.
-social_image: "https://downloads.thistleandmoth.org/reader-magnet/cover_social.jpg"
-
-# Direct link to the file in the public downloads bucket — shown as an
-# on-page button immediately after a successful signup, independent of the
-# welcome email (see layouts/partials/course-enroll-form.html). STUB — no
-# file uploaded yet.
-download_url: "https://downloads.thistleandmoth.org/reader-magnet/story.epub"
-download_text: "Download now"
-
-# Optional heading above the repeated signup form at the bottom of the page.
-repeat_cta_heading: "Want it?"
----
-TODO: reader-magnet page copy goes here.
-
-This page is a stub — the cover image and download link above won't
-resolve until real cover art and a real download file are uploaded to the
-`thistleandmoth-downloads` bucket, and this copy is replaced.