fix: implement lazy widget loading and store references

Defer widget creation from __init__ to compose() and store references
as instance variables to prevent garbage collection and initialization
order issues. This resolves ScreenStackError during TUI startup.

Changes:
- Move LogPane, GraphOverview, ChatRepl creation to compose()
- Store widgets as instance variables (self.log_pane, etc)
- Restore Horizontal/Vertical container layout

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

core/framework/tui/app.py

@@ -53,14 +53,18 @@ class AdenTUI(App):
     def __init__(self, runtime: AgentRuntime):
         super().__init__()
         self.runtime = runtime
-        self.log_pane = LogPane()
-        self.graph_view = GraphOverview()
-        self.chat_repl = ChatRepl(runtime)
+        # Widgets will be created lazily in compose()
 
     def compose(self) -> ComposeResult:
         """Create child widgets for the app."""
         yield Header()
 
+        # Lazy-load widgets here to avoid initialization order issues
+        # Store as instance variables to prevent garbage collection
+        self.log_pane = LogPane()
+        self.graph_view = GraphOverview()
+        self.chat_repl = ChatRepl(self.runtime)
+
         yield Horizontal(
             Container(self.graph_view, id="left-pane"),
             Vertical(