feat: add Ctrl+D cancellation to TUI

cmd/sidekick-tui/main.go

@@ -321,6 +321,20 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
       m.viewport.HalfViewUp()
       return m, nil
     case tea.KeyPgDown, tea.KeyCtrlD:
+      if msg.Type == tea.KeyCtrlD && m.isThinking {
+        if m.cancel != nil {
+          m.cancel()
+        }
+        m.isThinking = false
+        m.currentReasoning = ""
+        m.currentStream = ""
+        m.history = append(m.history, sidekick.Message{
+          Role:    sidekick.MessageRoleSystem,
+          Content: "Request cancelled.",
+        })
+        m.updateViewportContent()
+        return m, nil
+      }
       m.viewport.HalfViewDown()
       return m, nil
     case tea.KeyCtrlJ:
@@ -411,11 +425,13 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
   return m, tea.Batch(tiCmd, vpCmd)
 }
 
-func (m model) runAgent() tea.Cmd {
-  return func() tea.Msg {
-    res, err := m.agent.Run(context.Background(), m.history, m.pool)
-    return agentResponseMsg{response: res, err: err}
-  }
+func (m *model) runAgent() tea.Cmd {
+	var ctx context.Context
+	ctx, m.cancel = context.WithCancel(context.Background())
+	return func() tea.Msg {
+		res, err := m.agent.Run(ctx, m.history, m.pool)
+		return agentResponseMsg{response: res, err: err}
+	}
 }
 
 func (m model) View() string {