Replace posts/001/slug nesting with issue taxonomy

Use Hugo's `issue = "issues"` taxonomy so each post carries
`issues: ["01"]` in frontmatter. Issue pages live at /issues/01/
with a dedicated _index.md holding title, season, and description.
The /issues/ archive page uses data/issues.json for forthcoming
entries that have no posts yet.

Layouts:
- layouts/issues/list.html → plate grid for a single issue term
- layouts/issues/terms.html → issues archive (new)
- _default/single.html → scopes grid to issue term page, back-link to /issues/01/
- baseof.html → ribbon links to /issues/01/, JSON island includes issue field
- partials/header.html → Issues link → /issues/

JS:
- lbClose() navigates to /issues/${issueId}/ on a single-post page
- lightbox meta panel shows linked Issue fact

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

content/issues/01/_index.md

@@ -0,0 +1,7 @@
+---
+title: "Fabric, Light & Gesture"
+description: "The inaugural plates — one hundred photographs gathered across ateliers, streets, and quiet hotel corridors."
+issueNumber: "№ 01"
+season: "Spring MMXXVI"
+status: "current"
+---