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/gameplay/galaxy_creation.rs

@@ -0,0 +1,18 @@
+use bevy::prelude::*;
+
+// ── Markers ─────────────────────────────────────────────────────────────────
+
+#[derive(Component)]
+pub struct GalaxyCreationUi;
+
+// ── Galaxy Creation Screen ──────────────────────────────────────────────────
+
+pub fn setup_galaxy_creation(_commands: Commands) {
+    // TODO: spawn galaxy creation UI
+}
+
+pub fn despawn_galaxy_creation(mut commands: Commands, query: Query<Entity, With<GalaxyCreationUi>>) {
+    for entity in &query {
+        commands.entity(entity).despawn();
+    }
+}