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/trpc/routers/architect.ts

@@ -25,7 +25,7 @@ import type { PageRepository } from '../../db/repositories/page-repository.js';
 import type { Phase, Task } from '../../db/schema.js';
 import type { PageForSerialization } from '../../agent/content-serializer.js';
 
-async function gatherInitiativeContext(
+export async function gatherInitiativeContext(
   phaseRepo: PhaseRepository | undefined,
   taskRepo: TaskRepository | undefined,
   pageRepo: PageRepository | undefined,