feat: add persistent profile storage to ~/.kvs/config.json

Implemented roadmap #1: Configuration file for persistent profiles.

Features:
- Auto-saves profiles to ~/.kvs/config.json on add/use/remove
- Auto-loads profiles on shell startup
- File created with 0600 permissions for token security
- Shows active profile in welcome message
- Added 'profile save' and 'profile load' commands for manual control

Technical details:
- Created config.go with LoadConfig/SaveConfig functions
- Profile changes automatically trigger persistence
- ~/.kvs directory created with 0700 permissions if missing
- Gracefully handles missing config file on first run

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

main.go

@@ -11,6 +11,11 @@ import (
 func main() {
 	client := NewKVSClient("http://localhost:8090")
 
+	// Load saved profiles
+	if config, err := LoadConfig(); err == nil {
+		client.syncConfigToClient(config)
+	}
+
 	// Setup readline
 	rl, err := readline.NewEx(&readline.Config{
 		Prompt:          cyan("kvs> "),
@@ -26,6 +31,9 @@ func main() {
 
 	fmt.Println(magenta("KVS Interactive Shell"))
 	fmt.Println("Type 'help' for available commands")
+	if client.activeProfile != "" {
+		fmt.Println(green("Active profile:"), client.activeProfile)
+	}
 	fmt.Println()
 
 	for {