feat: allow branching from both user and assistant messages

- Relax branchSessionRecord validation to accept user or assistant roles
- Show "Branch from here" hover button on all conversation messages
- Contextual tooltip: "starting from this message" vs "continuing from this response"
- Add test for branching from assistant message
- Replace non-user rejection test with non-conversation rejection test

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/ChatPane.tsx

@@ -380,11 +380,13 @@ export function ChatPane({
                               {phaseLabel}
                             </span>
                           )}
-                          {isUser && onBranchFromMessage && (
+                          {onBranchFromMessage && (
                             <button
                               className="ml-auto flex items-center gap-1 rounded-md px-1.5 py-0.5 text-[11px] text-[var(--color-text-muted)] opacity-0 transition-all duration-150 hover:bg-[var(--color-surface-2)] hover:text-[var(--color-accent)] group-hover:opacity-100"
                               onClick={() => onBranchFromMessage(message.id)}
-                              title="Branch from here — create a new session starting from this message"
+                              title={isUser
+                                ? 'Branch from here — create a new session starting from this message'
+                                : 'Branch from here — create a new session continuing from this response'}
                               type="button"
                             >
                               <GitBranch className="size-3" />

src/shared/domain/sessionLibrary.ts

@@ -237,8 +237,8 @@ export function branchSessionRecord(
     throw new Error(`Message ${messageId} not found in session ${session.id}.`);
   }
 
-  if (sourceMessage.role !== 'user') {
-    throw new Error('Only user messages can be used as a branch point.');
+  if (sourceMessage.role !== 'user' && sourceMessage.role !== 'assistant') {
+    throw new Error('Only user or assistant messages can be used as a branch point.');
   }
 
   const branchedMessages = session.messages.slice(0, sourceMessageIndex + 1).map(cloneChatMessageRecord);