Rewrite TUI using page-based DSL

Each BBS screen is now a declarative `page` block. Shared rendering
logic (wiki list, scroll helpers, wiki page opening) lives in a `helpers`
block. The persistent chrome (header bar, sidebar menu, footer) is
defined once in a `chrome` block rendered every frame.

The main flow no longer reaches into runner internals: context flows
through naturally via the updated rubbs `tui` step.

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

Gemfile.lock

@@ -1,8 +1,8 @@
 GIT
   remote: https://git.teletype.hu/tools/rubbs.git
-  revision: 8a3e38aa25b29575b7038cdab753cad0a126c698
+  revision: cac10176a25eb5d917f559d4dfb6c7d9c0dc34da
   specs:
-    bbs (0.1.0)
+    bbs (0.2.0)
       artii (~> 2.1)
 
 GEM