Add alias completion in command mode

[yimsk]

Problem

After #65 (AWS CLI naming alignment), typing :cost and pressing Tab no longer autocompletes to costexplorer because the registered service name is now ce.

Currently, GetSuggestions() only returns registered service names, not aliases.

Expected Behavior

• :cost + Tab → suggests costexplorer (alias) or ce (service)
• :sfn + Tab → suggests stepfunctions
• Aliases should be included in completion candidates

Proposed Solution

1. Add GetAliases() method to registry
2. Include aliases in command_input.go GetSuggestions()

~20 lines change.