fix: MPS-24 Phase 2.8 — bulk tagger AJAX tag-focus + real drag-to-reorder

Operator (printableprompts.com, 481 products) reported the bulk tagger
'still refreshing the whole screen' and 'dragging tags doesn't work'
after 2.7. Two real defects the 2.7 static audit missed:

1. Tag-focus was a full-page navigation: clicking a tag chip is
   <a href=?focus=slug>, and the view loaded+rendered ALL products on
   EVERY GET. On a 481-product catalog every tag click reloaded a
   multi-MB page. The forms were AJAX; the dominant workflow was not.
2. Drag-to-reorder never existed: shop_tags.j2 shipped draggable=true +
   a handle + help text, but tag_bulk.js had ZERO drag handlers.

Fix:
- shop.py:shop_tags — all_products loads only when focus_tag or
  show_suggestions (bare GET is light). New AJAX branch: is_ajax +
  ?focus=slug -> JSON {focus, products:[{id,title,url,attached}]}.
- shop_tags.j2 — stable [data-focus-section] (always in DOM, hidden
  until focused); ?focus= chips carry data-tag-focus-link. No-JS
  unchanged (real navigation, server renders the section).
- tag_bulk.js — wireFocusLinks() intercepts chip clicks, fetchFocus()
  + renderFocus() swap the list in place, active-chip + history
  pushState/popstate, real-navigation fallback. wireDragAndDrop()
  HTML5 DnD -> persistOrder() POSTs action=set_order&tag_slugs=…
  (view already supported it) + re-syncs up/down disabled states.
  .tag-list-dragging CSS added.
- Tests: TestProductTagsSpa +4 (ajax focus json, unknown-slug null,
  set_order persists positions, bare GET no catalog). 1128 passed.

Docs: mps-24.md Phase 2.8, architecture.md, design-system.md, CLAUDE.md.
Deferred: AJAX 'Suggest categories' link (occasional click, not hot path).

CLAUDE.md

@@ -474,7 +474,16 @@ Form section: `home-layout-settings` (`views/shop.py`,
 stopwords inputs.
 
 Routes (registered before `shop_slug` catch-all):
-- `/s/{shop_id}/tags` — operator bulk tagger (`@shop_editor_required`)
+- `/s/{shop_id}/tags` — operator bulk tagger (`@shop_editor_required`).
+  **Phase 2.8**: tag-focus is AJAX — `tag_bulk.js` intercepts a
+  `[data-tag-focus-link]` chip click and fetches `?focus=<slug>` with
+  `X-Requested-With`; the view returns JSON
+  `{focus, products:[{id,title,url,attached}]}` and the JS swaps the
+  `[data-focus-section]` list in place (no full reload — critical on
+  large catalogs; the view only loads `all_products` when
+  `focus_tag or show_suggestions`). Real HTML5 drag-to-reorder on the
+  tag rows POSTs `action=set_order&tag_slugs=…`. No-JS unchanged: the
+  `?focus=` link is a real navigation, server still renders the section.
 - `/s/{shop_id}/tag/{slug}` — public tag detail page (works without JS)
 
 Tag input on product edit: comma-separated `tags` field on