Improve games command error message with helpful usage information

[Copilot]

Problem

When users run gabs games without any arguments, they receive a terse and unfriendly error message:

➜  GABS git:(main) ./gabs games
2025-09-13T20:20:20.297+0200	INFO	util/log.go:57	starting gabs	{"version": "0.1.0", "commit": "unknown", "built": "unknown", "subcmd": "games"}
games subcommand requires an action: list|add|remove|show

This message is technical and doesn't provide helpful guidance for users who want to understand what they can do with the games command.

Solution

This PR replaces the terse error message with a friendly, helpful response that shows users exactly what they can do:

➜  GABS git:(main) ./gabs games
2025-09-13T18:25:04.852Z	INFO	util/log.go:57	starting gabs	{"version": "0.1.0", "commit": "unknown", "built": "unknown", "subcmd": "games"}
Please specify what you'd like to do with games.

Game Management Commands:
  gabs games list               List all configured games
  gabs games add <id>           Add a new game configuration (interactive)
  gabs games remove <id>        Remove a game configuration
  gabs games show <id>          Show details for a game

Examples:
  gabs games list               # See all your configured games
  gabs games add minecraft      # Add a new game called 'minecraft'
  gabs games show minecraft     # View configuration for 'minecraft'
  gabs games remove minecraft   # Remove the 'minecraft' configuration

Changes Made

• Friendly error message: Changed from "games subcommand requires an action" to "Please specify what you'd like to do with games."
• Added dedicated usage helper: Created showGamesUsage() function that displays all available games commands with descriptions
• Practical examples: Included real-world examples showing how to use each command
• Minimal impact: Only modified the no-arguments error case; all existing functionality remains unchanged

The new approach follows the same user-friendly style as the main usage() function, providing clear guidance instead of just stating what's wrong.

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