Add Checkstyle linter

[Copilot]

Adds Checkstyle static analysis to enforce consistent Java coding standards across the project.

Configuration (config/checkstyle/)

• checkstyle.xml — Rule set covering naming conventions, import hygiene, public-field Javadoc, whitespace, modifier order, braces, and common coding pitfalls
• suppressions.xml — Suppresses all checks on bstats/ (auto-generated third-party code)

Build integration (app/build.gradle)

• Added built-in Gradle checkstyle plugin (Checkstyle 10.12.5)
• Configured to load rules from config/checkstyle/checkstyle.xml

Existing code fixes (PapermcPluginTemplatePlugin.java)

Two pre-existing violations corrected to pass the new checks:

• Modifier order: public final static → public static final
• Added Javadoc to all four public fields (instance, log, NAME, BSTATS_PLUGIN_ID)

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• checkstyle.org
  • Triggering command: /opt/hostedtoolcache/CodeQL/2.24.3/x64/codeql/tools/linux64/java/bin/java /opt/hostedtoolcache/CodeQL/2.24.3/x64/codeql/tools/linux64/java/bin/java -jar /opt/hostedtoolcache/CodeQL/2.24.3/x64/codeql/xml/tools/xml-extractor.jar --fileList=/tmp/codeql-scratch-d5ed3df9774d5cb6/dbs/java/working/files-to-index18094977859633374446.list --sourceArchiveDir=/tmp/codeql-scratch-d5ed3df9774d5cb6/dbs/java/src --outputDir=/tmp/codeql-scratch-d5ed3df9774d5cb6/dbs/java/trap/java (dns block)
• hub.spigotmc.org
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED -XX:MaxMetaspaceSize=384m -XX:+HeapDumpOnOutOfMemoryError -Xms256m -Xmx512m -Dfile.encoding=UTF-8 -Duser.country -Duser.language=en -Duser.variant -cp /home/REDACTED/.gradle/wrapper/dists/gradle-8.0.2-bin/25jlreiuz6u3xu2phlpa2vv4m/gradle-8.0.2/lib/gradle-launcher-8.0.2.jar org.gradle.launcher.daemon.bootstrap.GradleDaemon 8.0.2 (dns block)
• repo.papermc.io
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED -XX:MaxMetaspaceSize=384m -XX:+HeapDumpOnOutOfMemoryError -Xms256m -Xmx512m -Dfile.encoding=UTF-8 -Duser.country -Duser.language=en -Duser.variant -cp /home/REDACTED/.gradle/wrapper/dists/gradle-8.0.2-bin/25jlreiuz6u3xu2phlpa2vv4m/gradle-8.0.2/lib/gradle-launcher-8.0.2.jar org.gradle.launcher.daemon.bootstrap.GradleDaemon 8.0.2 (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)