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>

utils.go

@@ -99,6 +99,8 @@ var completer = readline.NewPrefixCompleter(
 		readline.PcItem("add"),
 		readline.PcItem("use"),
 		readline.PcItem("remove"),
+		readline.PcItem("save"),
+		readline.PcItem("load"),
 	),
 	readline.PcItem("get"),
 	readline.PcItem("put"),