Document -y/--yes flag in usage text

[Copilot]

The -y/--yes flag was added to skip confirmation prompts but wasn't documented in help output.

Changes

• Added echo_global_options() to centralize global flag documentation
• Updated echo_allcommand_usage() to show -y/--yes in syntax and options
• Updated echo_each_command_usage() to display global options section

Example

$ dots --help
Usage: dots [-y|--yes] install <Command> 
  
Global Options:   -y, --yes    Skip confirmation prompts 
  
Commands:  
...

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