tui: implement history navigation state initialization

cmd/sidekick-tui/main.go

@@ -55,6 +55,8 @@ type model struct {
   agent            *sidekick.Sidekick
   pool             map[string]*sidekick.Sidekick
   history          []sidekick.Message
+  historyIndex     int
+  promptDraft      string
   transientSteps   []string
   currentStream    string
   currentReasoning string
@@ -178,9 +180,10 @@ func initialModel() model {
   }
 
   return model{
-    textarea: ta,
-    agent:    agent,
-    pool:     pool,
+    textarea:     ta,
+    agent:        agent,
+    pool:         pool,
+    historyIndex: -1,
     history: []sidekick.Message{
       {Role: sidekick.MessageRoleSystem, Content: banner + "\n\nSidekick initialized. " + mcpStats + "\nType a message below."},
     },

cmd/sidekick-tui/tui_test.go

@@ -32,6 +32,13 @@ func TestInitialModel(t *testing.T) {
   }
 }
 
+func TestHistoryStateInitialization(t *testing.T) {
+  m := initialModel()
+  if m.historyIndex != -1 {
+    t.Errorf("expected historyIndex to be -1, got %d", m.historyIndex)
+  }
+}
+
 func TestModelUpdate(t *testing.T) {
   m := initialModel()
   m.ready = true