Make publish opt-out and fix the JVM mavenJava publication

[rasros]

What changed

• Added eignexPublish.publish (default true). Set it to false on internal modules that use the conventions but are never published — it skips the publication and signing wiring and the githubRepo requirement.
• Fixed the JVM publish branch in publish.gradle.kts: it referenced a mavenJava publication with named(...) but nothing ever created one, so com.eignex.jvm could never publish a plain JVM project. It now creates the publication from the java component.
• Upgraded the jvm-positive sample to apply the full com.eignex.jvm convention (it previously only applied com.eignex.lint), and the smoke test now publishes it to local staging so the mavenJava path has coverage.

Why

Putting an internal module (the klause benchmark harness) on com.eignex.jvm surfaced two gaps. First, the convention always forces publish wiring, which requires a githubRepo and is meaningless for a module that is never released. Second, the JVM half of the publish convention had never been exercised — every consumer so far is KMP, where the multiplatform plugin auto-creates publications — so the named("mavenJava") call failed with "Publication with name 'mavenJava' not found" on the first real JVM consumer. Both are now fixed, and the jvm-positive sample exercises the convention end to end so it does not regress.

Testing

Ran the full samples smoke test green: jvm-positive passes check and publishes mavenJava to local staging, kmp-positive runs wasmWasiNodeTest on the pinned node, cli-positive builds and runs, and the negative samples still fire their expected rules. Separately verified the opt-out by setting publish to false with no githubRepo — check still configures and passes and no publish tasks are created. The plugin build and the LintPluginRulesTest suite pass.