Phase 5: Update all content paths to SX expression URLs

- Update ~sx-doc :path values in docs.sx from old-style paths to SX
  expression URLs (fixes client-side rendered page nav resolution)
- Fix stale hrefs in content/pages.py code examples
- Fix tabs push-url in examples.sx
- Add self-defining-medium + sx-urls + sx-protocol to essay/plan cases

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

sx/sxc/examples.sx

@@ -508,7 +508,7 @@
     :sx-get (str "/(geography.(hypermedia.(example.(api.(tabs." tab ")))))")
     :sx-target "#tab-content"
     :sx-swap "innerHTML"
-    :sx-push-url (str "/geography/hypermedia/examples/tabs?tab=" tab)
+    :sx-push-url (str "/(geography.(hypermedia.(example.tabs)))?tab=" tab)
     :class (str "px-4 py-2 text-sm font-medium border-b-2 -mb-px transition-colors "
                 (if (= active "true")
                   "border-violet-600 text-violet-600"