Replace data/issues.json with content/_index.md files per issue

Issue metadata (title, description, season, status) now lives in
content/issues/XX/_index.md. terms.html reads from those pages via
$.Site.GetPage, driven by issueIds param in hugo.toml. Removes the
JSON data file and its generation from import-posts.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

content/issues/03/_index.md

@@ -0,0 +1,7 @@
+---
+title: "Atelier After Hours"
+description: "The slow work of cloth and thread, photographed when the studio is half-empty."
+issueNumber: "№ 03"
+season: "Autumn MMXXVI"
+status: "forthcoming"
+---