Support autocomplete/ghost text for extension slash command arguments

## Summary

Please add an SDK/API surface that lets Copilot CLI extensions provide autocomplete, ghost text, or structured argument suggestions for extension slash commands.

## Scenario

I am building a Copilot CLI extension that registers a deterministic `/cost` command via `joinSession({ commands: [...] })`. The command has useful subcommands/options such as:

```text
/cost
/cost help
/cost panel on|off|refresh
/cost session
/cost live-session
/cost --plan pro|pro-plus|business|enterprise
/cost --billing-model usage-based|premium-requests
/cost --currency USD
```

Built-in slash commands appear to show ghost text / inline hints for supported arguments as the user types. For extension commands, I only see the command name/description; after typing `/cost` there does not appear to be a way to suggest supported subcommands/options.

## Current SDK surface

The generated/types docs I found for `CommandDefinition` only expose:

```ts
export interface CommandDefinition {
name: string;
description?: string;
handler: CommandHandler;
}
```

That is enough to register `/cost`, but not enough to provide argument completions, examples, or dynamic suggestions.

## Requested capability

It would be useful if extension commands could declare one or more of the following:

- static usage text/examples for ghost text after the command name
- structured subcommands/options/enum values
- dynamic completion callback based on the current argument prefix and session context
- optional descriptions for each subcommand/option

For example, something like:

```ts
commands: [
{
name: 'cost',
description: 'Show Copilot session cost.',
usage: 'panel on|off|refresh | session | --plan ',
completions: async ({ args, cursor, sessionId }) => [
{ value: 'panel', description: 'Open or manage the cost panel' },
{ value: 'session', description: 'Show cost for a completed session' },
{ value: '--plan', description: 'Compare against a subscription plan' }
],
handler
}
]
```

## Why this matters

Extension slash commands can be deterministic command surfaces rather than model-routed prompts. Without argument hints, users have to remember command syntax or run a separate help command. Built-in-command-quality discoverability would make SDK extensions feel native and easier to learn.

## Workaround today

The extension can implement `/cost help`, but that is less discoverable and does not help while typing.

## Environment

Observed with Copilot CLI / SDK builds around `1.0.41-0` / `1.0.43` on Windows while authoring a project-scoped extension under `.github/extensions//extension.mjs`.

Commencez par la documentation de generated/types pour CommandDefinition et l’enregistrement des extensions illustré par joinSession({ commands: [...] }). Suivez la manière dont les commandes d’extension propres au projet sont gérées dans Copilot CLI. Done devrait fournir une surface SDK documentée pour l’utilisation statique ou les suggestions structurées, ainsi que pour la complétion dynamique des arguments, avec des descriptions lorsqu’elles sont prises en charge.

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