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>

hugo.toml

@@ -21,6 +21,7 @@ term     = ["HTML"]
 [params]
 description = "Roux is a slow-publishing fashion journal — one hundred photographs at a time. Editorial, couture, beauty and backstage plates."
 author      = "Roux Editorial"
+issueIds    = ["01", "02", "03"]
 issueNumber = "01"
 issueName   = "Fabric, Light & Gesture"
 issueSeason = "Spring MMXXVI"