Gradle plugin skeleton + markflow { } extension

[roxspring]

Implement the Gradle plugin entry point and configuration extension.

Scope

• Plugin class implementing Plugin<Project>
• Plugin ID: net.oxspring.markflow
• markflow { } extension block for project-level configuration (default pipeline, default output format, etc.)
• Plugin registers but does not configure any tasks — tasks are registered in subsequent issues
• Gradle module directory: gradle-plugin/, internal package: net.oxspring.markflow.gradle
• Module wired up with java-gradle-plugin and publication config from issue Versioning (Axion Release Plugin) + publication config #2

Acceptance criteria

• Applying the plugin to a project does not throw
• markflow { } extension block is accessible in build.gradle.kts
• Plugin ID is correct