Clean build: remove all dead code and fix warnings

- Remove unused fields (trigger_offset on Requesting, is_incomplete)
- Remove unused methods (selected_item, has_actions, is_menu_open,
  close_menu, move_selection, confirm_code_action, apply_workspace_edit)
- Remove unused import (Shrinkable in signature_help)
- Remove all #[allow(dead_code)] annotations
- Add build standards to AGENTS.md: zero warnings, zero errors required

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/code/signature_help.rs

@@ -4,8 +4,7 @@ use galaxy_core::ui::appearance::Appearance;
 use galaxy_core::ui::theme::color::internal_colors;
 use galaxyui::elements::{
     Border, ChildAnchor, ConstrainedBox, Container, CornerRadius, CrossAxisAlignment, Flex,
-    MainAxisSize, OffsetPositioning, ParentAnchor, ParentElement, ParentOffsetBounds, Radius,
-    Shrinkable, Text,
+    MainAxisSize, OffsetPositioning, ParentAnchor, ParentElement, ParentOffsetBounds, Radius, Text,
 };
 use galaxyui::{AppContext, Element, SingletonEntity, ViewContext};
 use lsp::{ParameterLabel, SignatureHelpResult, SignatureInfo};