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/ui/main_menu.rs

@@ -145,7 +145,7 @@ pub fn main_menu_buttons(
         if *interaction == Interaction::Pressed {
             match button {
                 MenuButton::ContinueGame => next_state.set(AppState::InGame), // Placeholder for now
-                MenuButton::NewGame => next_state.set(AppState::GalaxyCreation),
+                MenuButton::NewGame => next_state.set(AppState::Galaxy),
                 MenuButton::Options => next_state.set(AppState::Options),
                 MenuButton::Exit => {
                     exit.write(AppExit::Success);