Initial scaffold: submissions page + reader-magnet stub

Mirrors twosquirrelspress-site's submissions form and
michaelferrara-site's gift/course-enroll pattern. Gift page content,
cover art, and the download file are stubs until the real
reader-magnet is ready.

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

content/gift-thanks/index.md

@@ -0,0 +1,14 @@
+---
+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