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/drizzle/meta/_journal.json

@@ -190,6 +190,13 @@
       "when": 1771804800000,
       "tag": "0026_add_task_summary",
       "breakpoints": true
+    },
+    {
+      "idx": 27,
+      "version": "6",
+      "when": 1771891200000,
+      "tag": "0027_add_chat_sessions",
+      "breakpoints": true
     }
   ]
 }