Add 'help' command to display available commands

The game currently responds to unknown commands with a list of valid commands, but there's no dedicated help command. Players should be able to type help at any time to see available commands with brief descriptions.

**Acceptance criteria:**
- Add CommandType.Help to the enum
- CommandParser recognizes help and ? as help commands
- GameEngine.HandleHelp() prints a formatted list of all commands with short descriptions
- Use cyan ANSI color for the help text