Add page-based TUI DSL

Each screen is now a self-contained `page` block with its own `enter`,
`render`, and `key` handlers. Shared logic lives in a `helpers` block
mixed into the render context. A `chrome` block renders persistent UI
(header, sidebar, footer) before every frame.

New context primitives: `go(page)` for transitions, `reload` to re-run
the current page's `enter` block, `printable` key binding for text input.

`FlowRunner#run_tui` now passes the flow context to TUIRunner so username
and other session data are available without reaching into internals.

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

lib/bbs/flow_runner.rb

@@ -18,7 +18,7 @@ module BBS
       red:     "\e[1;31m",
     }.freeze
 
-    attr_reader :session_id
+    attr_reader :session_id, :session
 
     def initialize(session, session_id, flow)
       @session    = session
@@ -343,7 +343,7 @@ module BBS
     end
 
     def run_tui(step)
-      TUIRunner.new(@session, @session_id, step[:definition]).run
+      TUIRunner.new(@session, @session_id, step[:definition], context: @ctx).run
       nil
     rescue IOError, Errno::EPIPE, Errno::ECONNRESET
       :halt