feat(chat): add chat history persistence

Implement loading and saving of chat history to a JSON file. The history file path is configurable via Viper or defaults to ~/.config/grokkit/chat_history.json. Add functions to load, save, and get the history file path. Remove outdated comment in model selection.

cmd/chat.go

@@ -2,16 +2,63 @@ package cmd
 
 import (
 	"bufio"
+	"encoding/json"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 
 	"github.com/fatih/color"
 	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
 	"gmgauthier.com/grokkit/config"
 	"gmgauthier.com/grokkit/internal/grok"
 )
 
+type ChatHistory struct {
+	Messages []map[string]string `json:"messages"`
+}
+
+func loadChatHistory() []map[string]string {
+	histFile := getChatHistoryFile()
+	data, err := os.ReadFile(histFile)
+	if err != nil {
+		return nil
+	}
+
+	var hist ChatHistory
+	if err := json.Unmarshal(data, &hist); err != nil {
+		return nil
+	}
+
+	return hist.Messages
+}
+
+func saveChatHistory(messages []map[string]string) error {
+	histFile := getChatHistoryFile()
+	hist := ChatHistory{Messages: messages}
+	data, err := json.MarshalIndent(hist, "", " ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(histFile, data, 0644)
+}
+
+func getChatHistoryFile() string {
+	configFile := viper.GetString("chat.history_file")
+	if configFile != "" {
+		return configFile
+	}
+
+	home, _ := os.UserHomeDir()
+	if home == "" {
+		home = "."
+	}
+	histDir := filepath.Join(home, ".config", "grokkit")
+	_ = os.MkdirAll(histDir, 0755)
+	return filepath.Join(histDir, "chat_history.json")
+}
+
 var chatCmd = &cobra.Command{
 	Use:   "chat",
 	Short: "Interactive chat with Grok (use --agent for tool-enabled mode)",
@@ -37,7 +84,7 @@ func runChat(cmd *cobra.Command, args []string) {
 	if modelFlag != "" {
 		model = modelFlag
 	} else if agentMode {
-		model = config.GetModel("chat-agent", "") // we'll add this default next
+		model = config.GetModel("chat-agent", "")
 	} else {
 		model = config.GetModel("chat", "")
 	}