feat(tui): codex-style input, history, bang commands, single-line status bar

- Arrow-key input history (up/down with multiline awareness)
- Bang shell commands via ! prefix (requires --yolo)
- Codex-style input bar with mode indicator (R/Y/⌘)
- Command palette virtual scrolling
- Multiline input viewport fix
- Merged model pills + footer into single status line
- Removed shift+enter (wait for TUI library support)
- Removed default footer hints

internal/session/manager.go

@@ -288,9 +288,13 @@ func (m *Manager) providerNames() []string {
 	return names
 }
 
+func (m *Manager) AvailableModels() []string {
+	return append([]string(nil), m.provider.Models...)
+}
+
 func contains(values []string, value string) bool {
-	for _, item := range values {
-		if item == value {
+	for _, v := range values {
+		if v == value {
 			return true
 		}
 	}