Support autocomplete/ghost text for extension slash command arguments

Start with the generated/types documentation for CommandDefinition and the extension registration shown through joinSession({ commands: [...] }). Trace how project-scoped extension commands are handled in the Copilot CLI. Done should provide a documented SDK surface for static usage or structured suggestions and dynamic argument completion, with descriptions where supported.

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:

/cost
/cost help
/cost panel on|off|refresh
/cost session <session-id>
/cost live-session <session-id>
/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:

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:

commands: [
  {
    name: 'cost',
    description: 'Show Copilot session cost.',
    usage: 'panel on|off|refresh | session <session-id> | --plan <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/<name>/extension.mjs.