Registry of what a plugin declares it contributes, for observability and validation.

Three optional callbacks, registered the same convention-based way as Gamend.Hooks.KvSchemas — export them from the plugin's hooks module and they are picked up at load:

def notification_types do
  %{"quest_completed" => "Player finished a quest"}
end

def realtime_events do
  %{"quest_progress" => "Objective counter moved"}
end

def env_vars do
  [%{name: "MYGAME_DIFFICULTY", default: "normal", description: "Global difficulty"}]
end

notification_types/0 is enforced: Gamend.Notifications rejects a notification whose metadata["type"] is not declared by core or a plugin, so a client is never sent a code nobody documented. The other two are declarations only — a plugin can always read an env var directly, and events are validated at the push site — but they make the admin runtime page tell the whole truth instead of only core's half.