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/posts/poolside-sunset-meditation/index.md

@@ -3,6 +3,8 @@ title: "Poolside Sunset Meditation"
 description: "a fiery-haired model in a pink bikini sitting cross-legged in a serene pose beside a tropical pool at golden hour, blissful palm-tree editorial with radiant sunset sky"
 plate: "066"
 slug: "poolside-sunset-meditation"
+issues:
+  - "01"
 image: "poolside-sunset-meditation.png"
 weight: 66
 categories: