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/camera.rs

@@ -8,7 +8,7 @@ use crate::ui::util::cursor_over_ui;
 pub struct MainCamera;
 
 /// Orbit-style camera: rotates around `target` at `distance`, controlled by mouse.
-/// Used for inspection scenes like GalaxyCreation where there is no player to follow.
+/// Used for inspection scenes like Galaxy where there is no player to follow.
 ///
 /// Orientation is stored as a quaternion (`rotation`) to allow true 360° motion
 /// with no gimbal lock. The base orientation is camera at `target + (0, 0, distance)`