Migrate FightSystem modes and countdowns to GameCore

- Move shared arena-mode abstractions into GameCore
- Adapt fight countdowns and APIs to the shared model
- Add SteamWar issue audit workflow and SQL language settings

GameCore/README.md

@@ -0,0 +1,35 @@
+# GameCore
+
+GameCore is the Kotlin runtime library shared by SteamWar game-mode plugins.
+Its implementation is based on FightSystem, the most exhaustive game mode, and
+contains the mechanics that do not depend on a specific arena:
+
+- restartable Bukkit countdown scheduling, FightSystem milestone behavior,
+  message/sound/level presentation policy, and shared skip behavior;
+- generic team contracts, team lookup, readiness, and opposite-team handling;
+- reusable winner/draw results and score/last-remaining evaluation;
+- the complete state-dependent lifecycle (one-shots, tasks, listeners, commands,
+  and countdowns);
+- the FightSystem arena modes and typed JVM runtime properties;
+- observable, generic game-state transitions.
+
+GameCore intentionally does not define a game-state enum. Each consumer supplies
+its own enum to `GameStateMachine<S>` and the generic state-dependent classes, so
+each game mode controls its valid states while sharing the lifecycle behavior.
+
+```kotlin
+enum class State { WAITING, RUNNING, END }
+
+val lifecycle = GameStateMachine(State.WAITING)
+StateDependentListener(
+    lifecycle,
+    true,
+    setOf(State.RUNNING),
+    plugin,
+    ingameListener,
+)
+```
+
+FightSystem, MissileWars, TNTLeague, and TowerRun retain their own messages,
+inventories, listeners, schematics, and concrete win rules. They depend on the
+GameCore plugin at runtime; GameCore itself depends on KotlinCore.