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.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

npm-package/scripts/postinstall.js

@@ -73,8 +73,12 @@ function downloadFile(url, dest) {
       response.pipe(file);
 
       file.on('finish', () => {
-        file.close();
-        resolve();
+        // Wait for file.close() to complete before resolving
+        // This is critical on Windows where the file may still be locked
+        file.close((err) => {
+          if (err) reject(err);
+          else resolve();
+        });
       });
     });