Add NAWS terminal size negotiation and dynamic dimensions

- Negotiate NAWS (option 31) on connect to receive terminal cols/rows
- Expose term_cols/term_rows on Session (default 80×24)
- Add term_cols/term_rows helpers to TUIRunner::Context
- Make bar() default width dynamic via term_cols

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

lib/bbs/tui_runner.rb

@@ -56,13 +56,17 @@ module BBS
         @buf << "\e[#{row};#{col}H"
       end
 
+      def term_cols = @session.term_cols || 80
+      def term_rows = @session.term_rows || 24
+
       def text(content, x: nil, y: nil, style: nil)
         at(x, y) if x && y
         color = STYLES[style]
         @buf << (color ? "#{color}#{content}\e[0m" : content.to_s)
       end
 
-      def bar(y:, content:, width: 80, style: :muted)
+      def bar(y:, content:, width: nil, style: :muted)
+        width ||= term_cols
         text content.to_s.ljust(width), x: 1, y: y, style: style
       end