Feature Request: Argument auto-completion for skills / custom commands

Describe the feature or problem you'd like to solve

Skills can expose multiple parameters, but users currently need to remember them or look up the skill documentation before invocation. It would be useful for Copilot CLI to support PowerShell-style argument auto-completion for skills and custom commands.

Proposed solution

Proposed behavior

At minimum:

Auto-complete declared parameter names.
Support completion from partial parameter names.
Avoid suggesting already-used, non-repeatable parameters.
Derive completions directly from the skill's parameter/schema definition.

Optionally, parameter metadata could enable value completion:

file → filesystem completion
enum → allowed values
boolean → true / false
Why this is useful
Improves discoverability of skill capabilities.
Reduces typos and invalid arguments.
Avoids repeatedly checking skill documentation.
Makes parameter-rich skills feel like first-class CLI commands.
Lets skill authors define arguments once, with the CLI providing the completion experience automatically.

Example prompts or workflows

/deploy-service --environment --config <config.json>

Additional context

No response

まず、Copilot CLI の補完エントリーポイントと、呼び出しに使用される skill/custom command のパラメーターまたはスキーマ定義を特定します。部分的な引数がどのように解析されるかを追跡し、宣言されている未使用のパラメーターが候補として提示されること、およびオプション値の補完が指定されたメタデータ規則に従うことを確認します。