fix(pilot): clear messages in resetSessionDerivedState — was leaking across chats

Symptom: sidebar showed "User mjones got locked out … 0 messages" but the
conversation pane was rendering 2 messages from a different chat. The
task lane content matched what was displayed (so the AI was fine post-
prompt-sweep) — the leak was purely UI: messages from the previous chat
stayed on screen until the new chat's getSession returned.

selectChat resetSessionDerivedState() then awaits getSession before
calling setMessages(detail.conversation_messages). Between the reset
and that await, the prior chat's messages remain visible. handleNewChat
already had an explicit setMessages([]) call so it was unaffected;
selectChat did not.

Folded setMessages([]) into resetSessionDerivedState so any new chat-
switch entry point gets the wipe for free.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/pages/AssistantChatPage.tsx

@@ -272,12 +272,18 @@ export default function AssistantChatPage() {
     }
   }
 
-  // Single source of truth for "wipe every per-session task-lane state field"
-  // before switching to a different chat. Called from selectChat, handleNewChat,
-  // sendPrefill, and handleResumeNew so adding new lane-scoped state in future
+  // Single source of truth for "wipe every per-chat UI state field" before
+  // switching to a different chat. Called from selectChat, handleNewChat,
+  // sendPrefill, and handleResumeNew so adding new chat-scoped state in future
   // phases only requires touching this one helper. Forgetting to clear a field
-  // leaks the previous session's data into the new one (Phase 5 regression).
+  // leaks the previous chat's data into the new one — first noticed as a task
+  // lane regression (Phase 5), surfaced again as the conversation pane showing
+  // the previous chat's messages while the sidebar entry said "0 messages".
+  // `messages` belongs in here too: selectChat clears it asynchronously when
+  // getSession returns, but the gap between switching and that response is
+  // exactly when the previous chat's content stays visible.
   const resetSessionDerivedState = useCallback(() => {
+    setMessages([])
     setShowTaskLane(false)
     setActiveQuestions([])
     setActiveActions([])