feat: Gradle plugin — specCheck task

[hermanngeorge15]

Description

Create a Kotlin Gradle plugin that wraps spec-ci-plugin's validation pipeline as a specCheck Gradle task for JVM projects.

Acceptance Criteria

• Gradle plugin with specCi { specFile = file("CLAUDE.md"); failOn = FailOn.ERRORS } DSL
• ./gradlew specCheck runs the 4-step pipeline locally
• Published to Gradle Plugin Portal
• Works with Kotlin DSL build scripts

Milestone

v0.1.0