Mention @ in the placeholder message (#553)

## Test plan
1. runIde
2. see Cody tool window


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| <img width="632" alt="image"
src="https://github.com/sourcegraph/jetbrains/assets/19799111/5eaf06f4-dc3f-466b-8350-3b14e9dfe0c9">
| <img width="546" alt="image"
src="https://github.com/sourcegraph/jetbrains/assets/19799111/4d438ec0-43ec-4add-a449-2416782d9edc">
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src/main/java/com/sourcegraph/cody/PromptPanel.kt

@@ -19,6 +19,7 @@ import com.sourcegraph.cody.chat.CodyChatMessageHistory
 import com.sourcegraph.cody.chat.ui.SendButton
 import com.sourcegraph.cody.ui.AutoGrowingTextArea
 import com.sourcegraph.cody.vscode.CancellationToken
+import com.sourcegraph.common.CodyBundle
 import java.awt.Dimension
 import java.awt.event.*
 import java.io.File
@@ -48,7 +49,7 @@ class PromptPanel(project: Project, private val chatSession: ChatSession) : JLay
 
   init {
     /** Initialize view */
-    textArea.emptyText.text = "Ask a question about this code..."
+    textArea.emptyText.text = CodyBundle.getString("PromptPanel.ask-cody.message")
     scrollPane.border = EmptyBorder(JBUI.emptyInsets())
     scrollPane.background = UIUtil.getPanelBackground()
 
@@ -242,6 +243,10 @@ class PromptPanel(project: Project, private val chatSession: ChatSession) : JLay
     }
   }
 
+  fun updateEmptyTextAfterFirstMessage() {
+    textArea.emptyText.text = CodyBundle.getString("PromptPanel.ask-cody.follow-up-message")
+  }
+
   companion object {
     private const val CHAT_MESSAGE_HISTORY_CAPACITY = 100
     private val KEY_ENTER = KeyboardShortcut(KeyStroke.getKeyStroke(KeyEvent.VK_ENTER, 0), null)

src/main/kotlin/com/sourcegraph/cody/chat/ui/ChatPanel.kt

@@ -123,6 +123,7 @@ class ChatPanel(project: Project, chatSession: ChatSession) :
     if (messagesPanel.componentCount == 1) {
       modelDropdown.isEnabled = false
     }
+    promptPanel.updateEmptyTextAfterFirstMessage()
     messagesPanel.addOrUpdateMessage(message, index, shouldAddBlinkingCursor)
   }
 

src/main/resources/CodyBundle.properties

@@ -106,3 +106,5 @@ popup.select-chat=Select Chat
 popup.remove-chat=Remove Chat
 popup.remove-all-chats=Remove All Chats
 
+PromptPanel.ask-cody.message=Message (type @ to include specific files as context)
+PromptPanel.ask-cody.follow-up-message=Follow-up message (type @ to include specific files as context)