fix: edge selection highlighting and group-chat connection rules

- Add CSS styling for selected edges (indigo highlight with glow)
- Enable orchestrator↔agent connections in group-chat mode
- Downgrade disconnected-agent validation from error to warning
- Rename addHandoffEdge to addEdge (generic for all modes)
- Add tests for group-chat connection rules

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/pattern-graph/PatternGraphCanvas.tsx

@@ -22,7 +22,7 @@ import type { OrchestrationMode, PatternDefinition, PatternGraph } from '@shared
 import { resolvePatternGraph } from '@shared/domain/pattern';
 import type { ModelDefinition } from '@shared/domain/models';
 import {
-  addHandoffEdge,
+  addEdge,
   autoLayoutGraph,
   fromCanvasPositions,
   isConnectionAllowed,
@@ -143,7 +143,7 @@ function PatternGraphCanvasInner({
       }
 
       if (connection.source && connection.target) {
-        const updatedGraph = addHandoffEdge(graph, connection.source, connection.target);
+        const updatedGraph = addEdge(graph, connection.source, connection.target);
         onGraphChange(updatedGraph);
       }
     },