build: add Kotlin Toolchain migration with Gradle fallback

plugins/github-release/README.md

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+# KSLog GitHub release Toolchain plugin
+
+Kotlin Toolchain 0.13.0 has no built-in GitHub release task. This local plugin
+keeps the existing explicit release capability as a narrow wrapper around the
+standard `gh release create` CLI.
+
+It extracts the matching version section from `CHANGELOG.md`, preserves the
+legacy tag/title/target conventions, accepts the existing
+`GITHUB_RELEASE_TOKEN` variable plus standard `GH_TOKEN`/`GITHUB_TOKEN`, and
+supports a non-network `KSLOG_GITHUB_RELEASE_DRY_RUN=true` validation mode. It
+runs the committed release/version consistency preflight before rendering or
+creating a release. No CI workflow invokes it.
+
+`gh release create` does not modify local git state, but GitHub may create the
+remote tag at `--target` when the requested tag does not already exist. A real
+invocation therefore mutates both the remote release and potentially the remote
+tag; dry-run mode performs neither action.