feat: open subagent sessions read-only in context panel

Adds read-only embedded chat mode without hiding permission prompts
Opens subagent sessions in the context panel instead of replacing the main chat
Fixes context panel message loading for embedded sessions

packages/ui/src/components/chat/message/MessageBody.tsx

@@ -91,7 +91,8 @@ const normalizeSubtaskModel = (model: SubtaskPartLike['model']): string | null =
 
 const UserSubtaskPart: React.FC<{ part: SubtaskPartLike }> = ({ part }) => {
     const [expanded, setExpanded] = React.useState(false);
-    const setCurrentSession = useSessionUIStore((state) => state.setCurrentSession);
+    const effectiveDirectory = useEffectiveDirectory();
+    const openContextPanelTab = useUIStore((state) => state.openContextPanelTab);
     const { t } = useI18n();
 
     const description = typeof part.description === 'string' ? part.description.trim() : '';
@@ -151,7 +152,13 @@ const UserSubtaskPart: React.FC<{ part: SubtaskPartLike }> = ({ part }) => {
                         type="button"
                         className="typography-meta text-muted-foreground hover:text-foreground transition-colors underline underline-offset-2"
                         onClick={() => {
-                            void setCurrentSession(taskSessionID);
+                            if (!effectiveDirectory) return;
+                            openContextPanelTab(effectiveDirectory, {
+                                mode: 'chat',
+                                dedupeKey: `session:${taskSessionID}`,
+                                label: description || agent || t('contextPanel.mode.chat'),
+                                readOnly: true,
+                            });
                         }}
                     >
                         {t('chat.messageBody.subtask.openSession')}

packages/ui/src/components/chat/message/parts/ToolPart.tsx

@@ -1080,7 +1080,8 @@ const TaskToolSummary: React.FC<{
     isActive?: boolean;
 }> = ({ entries, isExpanded, isMobile, output, sessionId, onShowPopup, input, animateTailText = true, isActive = false }) => {
     const { t } = useI18n();
-    const setCurrentSession = useSessionUIStore((state) => state.setCurrentSession);
+    const currentDirectory = useDirectoryStore((state) => state.currentDirectory);
+    const openContextPanelTab = useUIStore((state) => state.openContextPanelTab);
     const showToolFileIcons = useUIStore((state) => state.showToolFileIcons);
     const displayEntries = entries;
 
@@ -1092,8 +1093,13 @@ const TaskToolSummary: React.FC<{
 
     const handleOpenSession = (event: React.MouseEvent) => {
         event.stopPropagation();
-        if (sessionId) {
-            setCurrentSession(sessionId);
+        if (sessionId && currentDirectory) {
+            openContextPanelTab(currentDirectory, {
+                mode: 'chat',
+                dedupeKey: `session:${sessionId}`,
+                label: agentType.charAt(0).toUpperCase() + agentType.slice(1),
+                readOnly: true,
+            });
         }
     };