Feature request: Support custom CLI aliases (similar to git)

### Describe the feature or problem you'd like to solve

Support custom CLI aliases (similar to git)

### Proposed solution

I want to be able to configure custom aliases, as is possible in git.
Configured either via `json` config file or CLI (`copilot alias btw ask`).

### Additional context

I like having custom aliases for git.
Some are simple 1-to-1 aliases that introduce shorter names for common commands (commit => ci, checkout => co, status => st).
Some are a bit more complex and pass some additional flags/values: `git config --global alias.pushu "push --set-upstream origin HEAD"`. (I know about `push.autoSetupRemote true`, this is just an example.)

It would be nice to have the option to configure custom aliases for Copilot, too.

Some people were already asking for specific aliases :
https://github.com/github/copilot-cli/issues/1421
https://github.com/github/copilot-cli/issues/357

In this issue, the person was asking for `/btw` command from Claude, but it was added as `/ask`. Would be nice to have the ability to configure it as an alias for people switching from Claude.
https://github.com/github/copilot-cli/issues/2078

The request concerns the CLI alias command (`copilot alias btw ask`) and JSON configuration; begin by locating the existing CLI command and config entry points. Done means users can define aliases in either supported form, including aliases with extra arguments, and invoke the configured names.

Rédigé par le modèle d'indexation à partir du texte de l'issue.