feat: Add persistent chat sessions for iterative phase/task refinement

Introduces a chat loop where users send instructions to an agent that
applies changes (create/update/delete phases, tasks, pages) and stays
alive for follow-up messages. Includes schema + migration, repository
layer, chat prompt, file-io action field extension, output handler chat
mode, revert support for deletes, tRPC procedures, events, frontend
slide-over UI with inline changeset display and revert, and docs.

apps/server/db/repositories/index.ts

@@ -72,3 +72,9 @@ export type {
   ConversationRepository,
   CreateConversationData,
 } from './conversation-repository.js';
+
+export type {
+  ChatSessionRepository,
+  CreateChatSessionData,
+  CreateChatMessageData,
+} from './chat-session-repository.js';