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/0027_add_chat_sessions.sql

@@ -0,0 +1,26 @@
+-- Chat sessions and messages for iterative phase/task refinement
+CREATE TABLE `chat_sessions` (
+	`id` text PRIMARY KEY NOT NULL,
+	`target_type` text NOT NULL,
+	`target_id` text NOT NULL,
+	`initiative_id` text NOT NULL REFERENCES `initiatives`(`id`) ON DELETE CASCADE,
+	`agent_id` text REFERENCES `agents`(`id`) ON DELETE SET NULL,
+	`status` text DEFAULT 'active' NOT NULL,
+	`created_at` integer NOT NULL,
+	`updated_at` integer NOT NULL
+);
+--> statement-breakpoint
+CREATE INDEX `chat_sessions_target_idx` ON `chat_sessions` (`target_type`, `target_id`);
+--> statement-breakpoint
+CREATE INDEX `chat_sessions_initiative_id_idx` ON `chat_sessions` (`initiative_id`);
+--> statement-breakpoint
+CREATE TABLE `chat_messages` (
+	`id` text PRIMARY KEY NOT NULL,
+	`chat_session_id` text NOT NULL REFERENCES `chat_sessions`(`id`) ON DELETE CASCADE,
+	`role` text NOT NULL,
+	`content` text NOT NULL,
+	`change_set_id` text REFERENCES `change_sets`(`id`) ON DELETE SET NULL,
+	`created_at` integer NOT NULL
+);
+--> statement-breakpoint
+CREATE INDEX `chat_messages_session_id_idx` ON `chat_messages` (`chat_session_id`);

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
     }
   ]
 }