--repo-config configuration ignored when .scala-steward.conf exists in project

[codergirlanna]

When both .scala-steward.conf (in the root of the project) and an additional custom configuration (via --repo-config) are provided, only the repository-specific configuration (.scala-steward.conf) is applied, while the custom configuration is ignored. This behavior is undocumented, leading to unexpected results.

In help.md:

--repo-config <uri>
      Additional repo config file (can be used multiple times)

For example:

.scala-steward.conf:

updates.allow = [
  { groupId = "org.scala-sbt", artifactId = "sbt" }
]

updates.allowPreReleases = [
  { groupId = "com....", artifactId = "xxx" }
]

custom.scala-steward.conf:

updates.allow = [
  { groupId = "com....", artifactId = "yyy" }
]

updates.allowPreReleases = [
  { groupId = "com....", artifactId = "yyy" }
]

Problem:
This behavior is not mentioned in the documentation, which creates confusion. It is expected that the custom configuration from --repo-config would be applied by complementing the project-specific .scala-steward.conf. Instead, the configuration in --repo-config is completely ignored, which makes it impossible to manage different library updates via running scala-steward with different configuration — for example, using only the project configuration for scheduled updates and the complemented custom configuration for on-demand runs.

Expected Behavior: The documentation should clarify this behavior, or ideally, Scala Steward should be updated to allow both .scala-steward.conf and the custom configuration provided via --repo-config to be applied by complementing each other.