feat(35-02): implement extractSubgraph for selection-aware context

- Implement extractSubgraph function with full selection logic
- Early return for no selection (return all nodes/edges, isScoped=false)
- Filter nodes into selected vs unselected with O(1) Set lookup
- Classify edges: fully within selection, boundary, or outside
- Build type breakdown from unselected nodes
- Identify boundary connections with direction (incoming/outgoing)
- All 8 tests passing (GREEN phase)

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

src/lib/chat/subgraphExtractor.ts

@@ -49,6 +49,88 @@ export function extractSubgraph(
   edges: WorkflowEdge[],
   selectedNodeIds: string[]
 ): SubgraphResult {
-  // TODO: Implement
-  throw new Error("Not implemented");
+  // Early return for no selection - return everything
+  if (selectedNodeIds.length === 0) {
+    return {
+      selectedNodes: nodes,
+      selectedEdges: edges,
+      restSummary: null,
+      isScoped: false,
+    };
+  }
+
+  // Build selectedSet for O(1) lookup
+  const selectedSet = new Set(selectedNodeIds);
+
+  // Filter nodes into selected vs unselected
+  const selectedNodes: WorkflowNode[] = [];
+  const unselectedNodes: WorkflowNode[] = [];
+
+  for (const node of nodes) {
+    if (selectedSet.has(node.id)) {
+      selectedNodes.push(node);
+    } else {
+      unselectedNodes.push(node);
+    }
+  }
+
+  // Filter edges: both endpoints in selectedSet -> selectedEdges
+  const selectedEdges: WorkflowEdge[] = [];
+  const boundaryEdges: WorkflowEdge[] = [];
+
+  for (const edge of edges) {
+    const sourceSelected = selectedSet.has(edge.source);
+    const targetSelected = selectedSet.has(edge.target);
+
+    if (sourceSelected && targetSelected) {
+      // Both endpoints selected -> fully within selection
+      selectedEdges.push(edge);
+    } else if (sourceSelected || targetSelected) {
+      // Exactly one endpoint selected -> boundary edge
+      boundaryEdges.push(edge);
+    }
+    // else: neither selected -> ignore
+  }
+
+  // Build type breakdown from unselected nodes
+  const typeBreakdown: Record<string, number> = {};
+  for (const node of unselectedNodes) {
+    const type = node.type;
+    typeBreakdown[type] = (typeBreakdown[type] || 0) + 1;
+  }
+
+  // Build boundaryConnections from boundary edges with direction
+  const boundaryConnections: BoundaryConnection[] = boundaryEdges.map((edge) => {
+    const sourceSelected = selectedSet.has(edge.source);
+    const targetSelected = selectedSet.has(edge.target);
+
+    if (targetSelected) {
+      // Target is selected, source is not -> incoming
+      return {
+        direction: "incoming" as const,
+        selectedNodeId: edge.target,
+        otherNodeId: edge.source,
+        handleType: edge.targetHandle || "unknown",
+      };
+    } else {
+      // Source is selected, target is not -> outgoing
+      return {
+        direction: "outgoing" as const,
+        selectedNodeId: edge.source,
+        otherNodeId: edge.target,
+        handleType: edge.sourceHandle || "unknown",
+      };
+    }
+  });
+
+  return {
+    selectedNodes,
+    selectedEdges,
+    restSummary: {
+      nodeCount: unselectedNodes.length,
+      typeBreakdown,
+      boundaryConnections,
+    },
+    isScoped: true,
+  };
 }