feat: enable snapshot releases to Sonatype snapshots repo

[rasros]

Summary

• Add conditional publishing logic to release workflow
• Publish final releases (v1.0.0) to Maven Central Portal
• Publish snapshot/pre-release versions (v1.0.0-SNAPSHOT, v1.0.0-RC1) to Sonatype snapshots (unlimited quota, no rate limits)

Motivation

Allows testing and cross-repo dependencies on snapshots without consuming Maven Central release quota.

[rasros]

Pushed a fix: the snapshot step called publishToSonatypeSnapshots, which does not exist. maven-publish derives the task from the repository name, so sonatypeSnapshots yields publishAllPublicationsToSonatypeSnapshotsRepository. Verified in kbuild, where it published 1.2.8-SNAPSHOT successfully.

Still blocked: the sonatypeSnapshots repository is declared in kbuild's publish.gradle.kts, and this repo pins com.eignex.kmp at 1.2.7, which does not declare it. The task will not resolve until kbuild ships a release containing it and the pin is bumped.