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I don't understand why I'd need to declare repos in pluginManagement, since I'm using the dependency as a regular project dependency, not as an applied plugin

Maybe it's required to build your included build?

Yep Main build • includes build of settings plugin ◦ settings plugin has a regular dependency on AGP • applies settings plugin

the transitive dependencies of your settings plugin are seen as "plugin" dependencies from your main build

Neat trick! But I'm not sure how to make it work with a build that contributes a Settings plugin. With 1, I get the same "Could not resolve AGP" error, but I can't use 2 because it's a Settings plugin ("Plugin with id 'plugin' not found.")

This seems to work:

pluginManagement {
  includeBuild("settings-plugin-included-build")
  apply(from = "./gradle/repositories2.gradle.kts")
}

repositories2.gradle.kts:

listOf(pluginManagement.repositories, dependencyResolutionManagement.repositories).forEach {
  it.apply {
    mavenCentral()
    google()
    gradlePluginPortal {
      content {
        includeModule("org.gradle.kotlin.embedded-kotlin", "org.gradle.kotlin.embedded-kotlin.gradle.plugin")
        includeGroup("org.gradle.kotlin")
        includeModule("me.champeau.gradle", "japicmp-gradle-plugin")
        includeModule("com.gradle.publish", "plugin-publish-plugin")
        includeModule("com.github.ben-manes", "gradle-versions-plugin")
      }
    }
    @Suppress("DEPRECATION")
    jcenter {
      content {
        // <https://github.com/Kotlin/kotlinx-nodejs/issues/16>
        includeModule("org.jetbrains.kotlinx", "kotlinx-nodejs")
      }
    }
  }
}

That does work, thank you!

Ah, interesting!