fix: use theme amber colors for gate state instead of arbitrary hex

Use --amber (#C6820E) and --amber-bg (#FDF6E9) from the design system
theme instead of hardcoded #D97706/#FFFBEB.

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

ui/src/components/ProcessDiagram/CompoundNode.tsx

@@ -69,7 +69,8 @@ export function CompoundNode({
   // Gate state: filter rejected or idempotent duplicate → amber container
   const ownState = node.id ? executionOverlay?.get(node.id) : undefined;
   const isGated = ownState?.filterMatched === false || ownState?.duplicateMessage === true;
-  const GATE_COLOR = '#D97706'; // amber-600
+  const GATE_COLOR = '#C6820E'; // --amber
+  const GATE_BG = '#FDF6E9';   // --amber-bg
   const effectiveColor = isGated ? GATE_COLOR : color;
 
   // _TRY_BODY / _CB_MAIN: transparent wrapper — no header, no border, just layout
@@ -125,7 +126,7 @@ export function CompoundNode({
   }
 
   // Default compound rendering (DO_TRY, EIP_CHOICE, EIP_FILTER, EIP_IDEMPOTENT_CONSUMER, etc.)
-  const containerFill = isGated ? '#FFFBEB' : 'white'; // amber-50 tint when gated
+  const containerFill = isGated ? GATE_BG : 'white';
   return (
     <g data-node-id={node.id} transform={`translate(${x}, ${y})`}>
       {/* Container body */}