feat(chat): add hydrateMessages, chat.history, and custom actions

Backend-controlled message history via hydrateMessages hook — loads from
  DB on every turn, replacing the linear accumulator. Imperative chat.history
  API (rollbackTo, remove, replace, slice) for modifying history from any
  hook or run(). Custom actions via actionSchema + onAction — typed actions
  sent through transport.sendAction() that wake the agent, modify state,
  then trigger run(). Adds sendAction to both TriggerChatTransport and
  AgentChat.

Author: ericallam