Add comprehensive PRD and terminal UX improvements

- Add PRD.md with detailed product requirements documentation
- Unify tmux screen size presets to consistent 24-row height
- Add Ctrl+Down keyboard shortcut to scroll terminal to bottom

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/src/spiceflow/adapters/tmux.clj

@@ -339,10 +339,10 @@
 
 ;; Screen size presets for different device orientations
 (def ^:private screen-sizes
-  {:fullscreen {:width 180 :height 60}
-   :desktop    {:width 100 :height 40}
+  {:fullscreen {:width 180 :height 24}
+   :desktop    {:width 100 :height 24}
    :landscape  {:width 65  :height 24}
-   :portrait   {:width 40  :height 35}})
+   :portrait   {:width 40  :height 24}})
 
 (defn resize-session
   "Resize a tmux session window to a preset size.