feat(tooling): fetch-authlib.sh to download authlib-injector.jar

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>

.claude/skills/deploy/SKILL.md

@@ -37,6 +37,7 @@ invoking this skill is the authorization to proceed, but:
      cd /home/ubuntu/mc/ulicraft-server-v1
      git pull --ff-only
      tooling/render-config.sh
+     tooling/fetch-authlib.sh
      docker compose down --remove-orphans
      docker compose up -d --build'
    ```
@@ -45,6 +46,9 @@ invoking this skill is the authorization to proceed, but:
    - `render-config.sh` re-renders drasl/nmsr/packwiz configs from `.env`
      (needs `.env` complete: `BASE_DOMAIN`, `RCON_PASSWORD`,
      `DISTRIBUTION_WEB_ROOT`, `CADDY_HTTP_*`).
+   - `fetch-authlib.sh` ensures `runtime/authlib-injector.jar` exists (gitignored;
+     skips if already valid). Missing jar = minecraft crashloops with "Error
+     opening zip file or JAR manifest missing".
 
 3. **Verify.**
    ```bash