fix: exclude subagent chats from sidebar pagination (#24404)

GetChats now returns only root chats (parent_chat_id IS NULL).
A new GetChildChatsByParentIDs query fetches children for visible
roots and embeds them in each parent's Children field. The
singular getChat endpoint does the same.

Archive invariant is one-way: parent archived implies child
archived. Parent archive/unarchive cascades via root_chat_id.
Individual child archive is permitted; child unarchive while the
parent is archived is rejected atomically (row lock on child,
re-read parent inside the transaction). Embedded children are
filtered by the caller's archive state so individually-archived
children stay hidden from active-parent views.

Gitsync MarkStale uses GetChatsByWorkspaceIDs directly;
MarkStaleParams.OwnerID removed (dead after the switch).

Frontend: buildChatTree reads from the embedded children field,
WebSocket handlers route child events into the parent's children
array, and archiving a child strips it from the parent cache.

coderd/database/queries.sql.go

@@ -6620,6 +6620,11 @@ WHERE
         WHEN $4::jsonb IS NOT NULL THEN chats.labels @> $4::jsonb
         ELSE true
     END
+    -- Paginate over root chats only. Children are fetched
+    -- separately via GetChildChatsByParentIDs and embedded under
+    -- each parent. Other callers that need the full set should
+    -- use a narrower query (e.g. GetChatsByWorkspaceIDs).
+    AND chats.parent_chat_id IS NULL
     -- Authorize Filter clause will be injected below in GetAuthorizedChats
     -- @authorize_filter
 ORDER BY
@@ -6838,6 +6843,95 @@ func (q *sqlQuerier) GetChatsUpdatedAfter(ctx context.Context, updatedAfter time
 	return items, nil
 }
 
+const getChildChatsByParentIDs = `-- name: GetChildChatsByParentIDs :many
+SELECT
+    chats.id, chats.owner_id, chats.workspace_id, chats.title, chats.status, chats.worker_id, chats.started_at, chats.heartbeat_at, chats.created_at, chats.updated_at, chats.parent_chat_id, chats.root_chat_id, chats.last_model_config_id, chats.archived, chats.last_error, chats.mode, chats.mcp_server_ids, chats.labels, chats.build_id, chats.agent_id, chats.pin_order, chats.last_read_message_id, chats.last_injected_context, chats.dynamic_tools, chats.organization_id, chats.plan_mode, chats.client_type,
+    EXISTS (
+        SELECT 1 FROM chat_messages cm
+        WHERE cm.chat_id = chats.id
+            AND cm.role = 'assistant'
+            AND cm.deleted = false
+            AND cm.id > COALESCE(chats.last_read_message_id, 0)
+    ) AS has_unread
+FROM
+    chats
+WHERE
+    chats.parent_chat_id = ANY($1 :: uuid[])
+    AND CASE
+        WHEN $2 :: boolean IS NULL THEN true
+        ELSE chats.archived = $2 :: boolean
+    END
+ORDER BY
+    chats.created_at ASC,
+    chats.id ASC
+`
+
+type GetChildChatsByParentIDsParams struct {
+	ParentIds []uuid.UUID  `db:"parent_ids" json:"parent_ids"`
+	Archived  sql.NullBool `db:"archived" json:"archived"`
+}
+
+type GetChildChatsByParentIDsRow struct {
+	Chat      Chat `db:"chat" json:"chat"`
+	HasUnread bool `db:"has_unread" json:"has_unread"`
+}
+
+// Fetches child chats of the given parents, optionally filtered by
+// archive state (NULL = all, true/false = match). The archive
+// invariant (parent archived implies child archived) is enforced
+// at write time, not here.
+func (q *sqlQuerier) GetChildChatsByParentIDs(ctx context.Context, arg GetChildChatsByParentIDsParams) ([]GetChildChatsByParentIDsRow, error) {
+	rows, err := q.db.QueryContext(ctx, getChildChatsByParentIDs, pq.Array(arg.ParentIds), arg.Archived)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []GetChildChatsByParentIDsRow
+	for rows.Next() {
+		var i GetChildChatsByParentIDsRow
+		if err := rows.Scan(
+			&i.Chat.ID,
+			&i.Chat.OwnerID,
+			&i.Chat.WorkspaceID,
+			&i.Chat.Title,
+			&i.Chat.Status,
+			&i.Chat.WorkerID,
+			&i.Chat.StartedAt,
+			&i.Chat.HeartbeatAt,
+			&i.Chat.CreatedAt,
+			&i.Chat.UpdatedAt,
+			&i.Chat.ParentChatID,
+			&i.Chat.RootChatID,
+			&i.Chat.LastModelConfigID,
+			&i.Chat.Archived,
+			&i.Chat.LastError,
+			&i.Chat.Mode,
+			pq.Array(&i.Chat.MCPServerIDs),
+			&i.Chat.Labels,
+			&i.Chat.BuildID,
+			&i.Chat.AgentID,
+			&i.Chat.PinOrder,
+			&i.Chat.LastReadMessageID,
+			&i.Chat.LastInjectedContext,
+			&i.Chat.DynamicTools,
+			&i.Chat.OrganizationID,
+			&i.Chat.PlanMode,
+			&i.Chat.ClientType,
+			&i.HasUnread,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Close(); err != nil {
+		return nil, err
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const getLastChatMessageByRole = `-- name: GetLastChatMessageByRole :one
 SELECT
     id, chat_id, model_config_id, created_at, role, content, visibility, input_tokens, output_tokens, total_tokens, reasoning_tokens, cache_creation_tokens, cache_read_tokens, context_limit, compressed, created_by, content_version, total_cost_micros, runtime_ms, deleted, provider_response_id