The server JVM agent (runtime/authlib-injector.jar, mounted at /extras) is
gitignored and was previously fetched by the deleted build-stack.sh prep. A
fresh checkout had no jar, so minecraft crashlooped with "Error opening zip
file or JAR manifest missing". Add a standalone fetch tool: resolves the latest
release asset, downloads, and verifies it's a real zip/jar (PK magic) before
trusting it. Idempotent (skips if valid, --force to re-download). Wire it into
the deploy skill and README launch steps.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
