fix: save placeholder message when task is cancelled

When user cancels a task with /stop, the user message was saved but
the assistant response was not, causing orphaned user messages in
session history. Now saves '[Task cancelled by user]' placeholder to
keep conversation history complete.

pkg/agent/loop.go

@@ -846,6 +846,12 @@ func (al *AgentLoop) runAgentLoop(
 	// 3. Run LLM iteration loop
 	finalContent, iteration, err := al.runLLMIteration(ctx, agent, messages, opts)
 	if err != nil {
+		// If task was cancelled, save a placeholder message to session
+		// This prevents the user message from being orphaned without a response
+		if errors.Is(err, context.Canceled) {
+			agent.Sessions.AddMessage(opts.SessionKey, "assistant", "[Task cancelled by user]")
+			agent.Sessions.Save(opts.SessionKey)
+		}
 		return "", err
 	}