fix: move session lifecycle actions to header bar in AssistantChatPage

- Add persistent session header with title, status badge, Resolve,
  Escalate, and Update Ticket/Share Update buttons — mirrors
  FlowPilotSessionPage pattern exactly
- Update Ticket label when psa_ticket_id present, Share Update otherwise
- Full mobile support via ⋯ overflow menu (Resolve, Escalate, Update, Pause)
- Strip _(not yet completed)_ markers from stored conversation_messages
  in unified_chat_service to prevent stale task lane items from prior
  turns leaking into new sessions via the AI's re-include instruction
- Add currentChatRef guard to handleResumeNew (was missing unlike handleSend)
- Remove Update/Conclude from chatbar — toolbar is now input utilities only

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/unified_chat_service.py

@@ -237,8 +237,10 @@ async def send_chat_message(
             ai_content, input_tokens, output_tokens = await _call_ai(**prompt_args)
 
             # Update branch conversation
+            # Strip _(not yet completed)_ markers before storage (same reason as main path)
+            stored_message = message.replace("_(not yet completed)_", "(pending)").replace("_(skipped)_", "(skipped)")
             msgs = list(branch.conversation_messages or [])
-            msgs.append({"role": "user", "content": message})
+            msgs.append({"role": "user", "content": stored_message})
             msgs.append({"role": "assistant", "content": ai_content})
             branch.conversation_messages = msgs
 
@@ -350,8 +352,14 @@ async def send_chat_message(
     # Store DISPLAY content (markers stripped) in conversation_messages.
     # The format reminder in the user message + system prompt final reminder
     # are sufficient to keep the AI emitting markers on subsequent turns.
+    #
+    # Strip _(not yet completed)_ task markers from the stored user message.
+    # The AI processes them correctly on the current turn, but persisting them
+    # into history causes the AI to re-inject stale task lane items from prior
+    # turns — even across unrelated topics in a long session.
+    stored_message = message.replace("_(not yet completed)_", "(pending)").replace("_(skipped)_", "(skipped)")
     msgs = list(session.conversation_messages or [])
-    msgs.append({"role": "user", "content": message})
+    msgs.append({"role": "user", "content": stored_message})
     msgs.append({"role": "assistant", "content": display_content})
     session.conversation_messages = msgs
     session.step_count += 2  # message count for display