Split Rust game src into modules; update AGENTS.md for dual toolchain

- Refactor apps/game/src/main.rs into modules: state.rs, camera.rs, ui/, gameplay/
- Add gameplay/galaxy_creation.rs scaffold (stubs only, no new logic)
- Update root AGENTS.md: separate TS workspace vs Rust game commands
- Update apps/AGENTS.md: docs/game/site use two toolchains, not one
- Add apps/game/AGENTS.md: build commands, module layout, naming conventions, Plugin pattern

apps/game/src/state.rs

@@ -0,0 +1,11 @@
+use bevy::prelude::*;
+
+#[derive(Clone, Eq, PartialEq, Debug, Hash, States, Default)]
+pub enum AppState {
+    #[default]
+    MainMenu,
+    GalaxyCreation,
+    CharacterCreation,
+    InGame,
+    Options,
+}