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>

cmd_system.go

@@ -27,10 +27,12 @@ KVS Interactive Shell - Available Commands:
 Connection & Authentication:
   connect <url>              - Connect to KVS server
   auth <token>               - Set authentication token
+  profile                    - List all profiles
   profile add <name> <token> <user-uuid> [url] - Add user profile
   profile use <name>         - Switch to user profile
   profile remove <name>      - Remove user profile
-  profile                    - List all profiles
+  profile save               - Save profiles to ~/.kvs/config.json
+  profile load               - Load profiles from ~/.kvs/config.json
 
 Key-Value Operations:
   get <key>                  - Retrieve value for key