fix(graph,npm): fix nil pointer crash and Windows installer lock

- graph.go: Pass subgraph to renderGraph() instead of nil (fixes #657)
  The nil subgraph was dereferenced in computeDependencyCounts()
  causing a panic when running `bd graph` on epics.

- postinstall.js: Wait for file.close() callback before resolving (fixes #652)
  On Windows, the file was still locked when extraction started
  because resolve() was called immediately after file.close()
  without waiting for the close to complete.

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

cmd/bd/graph.go

@@ -99,7 +99,7 @@ Colors indicate status:
 		}
 
 		// Render ASCII graph
-		renderGraph(layout, nil)
+		renderGraph(layout, subgraph)
 	},
 }