Rename galaxy_creation to galaxy; add character creation skeleton

- Rename GalaxyCreationPlugin -> GalaxyPlugin, AppState::GalaxyCreation
  -> AppState::Galaxy, module path galaxy_creation -> galaxy (folder
  rename preserves git history via git mv).
- Rename GalaxyCreationSpawned -> GalaxySpawned, despawn_galaxy_creation
  -> despawn_galaxy.
- Update AGENTS.md module layout, plugin pattern example, and naming
  table for the new names.
- Add apps/game/src/gameplay/character_creation/ skeleton: placeholder
  UI with Confirm (-> InGame) and Back (-> Galaxy) buttons plus Escape
  shortcut. Wired into main.rs via CharacterCreationPlugin.

apps/game/src/state.rs

@@ -4,7 +4,7 @@ use bevy::prelude::*;
 pub enum AppState {
     #[default]
     MainMenu,
-    GalaxyCreation,
+    Galaxy,
     CharacterCreation,
     InGame,
     Options,