test(security): add createSecurityTaskRunner mock to securityScanSpec

securityScanSpec provides a custom validateFileSafety but pack() still
calls createSecurityTaskRunner before passing the prewarm runner through.
Add a no-op mock so the test doesn't try to spawn real worker threads.

https://claude.ai/code/session_017gxajnSibkNqKmPDG8wDBp

tests/core/security/securityScanSpec.test.ts

@@ -89,6 +89,10 @@ const runPack = async (rootDir: string, config: RepomixConfigMerged) =>
       taskRunner: { run: async () => 0, cleanup: async () => {} },
       warmupPromise: Promise.resolve(),
     }),
+    createSecurityTaskRunner: () => ({
+      taskRunner: { run: async () => [], cleanup: async () => {} },
+      warmupPromise: Promise.resolve(),
+    }),
     calculateMetrics: async (processedFiles) => ({
       totalFiles: processedFiles.length,
       totalCharacters: processedFiles.reduce((acc, f) => acc + f.content.length, 0),