fix: correct file permissions in integration test (gosec G306)

Change WriteFile permissions from 0o644 to 0o600 to address security
linting issue about file permissions being too permissive.

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

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

internal/cli/integration_test.go

@@ -2144,7 +2144,7 @@ func copyFile(src, dst string) error {
 		return err
 	}
 
-	err = os.WriteFile(dst, srcData, 0o644)
+	err = os.WriteFile(dst, srcData, 0o600)
 	if err != nil {
 		return err
 	}