feat(cli): add subcommand completion to generate various shell completions

[kskarthik]

Summary

Hy again! This PR enables a CLI feature to generate completions for all system shells: bash, elvish, fish, powershell, zsh using the clap_complete crate https://docs.rs/clap_complete/latest/clap_complete/

Shell completions help users by suggesting command names, options, and values while typing in the command line, making it easier to discover and use commands without needing to remember exact syntax.

Type of Change

• Feature
• Bug fix
• Refactor / Code quality
• Performance improvement
• Documentation
• Tests
• Security fix
• Build / Release
• Other (specify below)

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

Tested locally. Build successful

Usage

goose completion --help

Generate the autocompletion script for the specified shell

Usage: goose completion <SHELL>

Arguments:
  <SHELL>  [possible values: bash, elvish, fish, powershell, zsh]

Options:
  -h, --help  Print help

[jamadeo]

nice!