URL restructure, 404 page, trailing slash normalization, layout fixes

- Rename /reactive-islands/ → /reactive/, /reference/ → /hypermedia/reference/,
  /examples/ → /hypermedia/examples/ across all .sx and .py files
- Add 404 error page (not-found.sx) working on both server refresh and
  client-side SX navigation via orchestration.sx error response handling
- Add trailing slash redirect (GET only, excludes /api/, /static/, /internal/)
- Remove blue sky-500 header bar from SX docs layout (conditional on header-rows)
- Fix 405 on API endpoints from trailing slash redirect hitting POST/PUT/DELETE
- Fix client-side 404: orchestration.sx now swaps error response content
  instead of silently dropping it
- Add new plan files and home page component

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

sx/sxc/handlers/examples.sx

@@ -89,7 +89,7 @@
         (range start (+ start 5)))
       (if (<= next 6)
         (div :id "scroll-sentinel"
-          :sx-get (str "/examples/api/scroll?page=" next)
+          :sx-get (str "/hypermedia/examples/api/scroll?page=" next)
           :sx-trigger "intersect once"
           :sx-target "#scroll-items"
           :sx-swap "beforeend"