Hook event for slash-command / skill invocation (like Claude Code's UserPromptExpansion), and async command hooks #5305

Describe the feature or problem you'd like to solve

Add a hook event that fires when a slash command or skill is invoked, with a matcher on its name

Proposed solution

We ship skills as plugins for both Claude Code and Copilot CLI and log their use from plugin hooks. A skill the user types (/my-skill) doesn't go through the model's skill tool, so the only way to see it today is a userPromptSubmitted hook. That hook starts a process on every prompt and blocks it (~0.3 s), just to check whether the prompt starts with /.

Claude Code solves this with a UserPromptExpansion event. It fires when a typed command expands, carries command_name (plugin:skill) and command_source, and supports a matcher on the command name, so the hook only runs for matching commands.

Requests:

1. A hook event (e.g. skillInvoked / userPromptExpansion) that fires when a typed slash command or skill expands, with the plugin and skill name in the payload and a matcher on the name.
2. Support for "async": true on command hooks, so a logging hook doesn't delay the prompt (Claude Code runs async hooks in the background; Copilot CLI 1.0.105 currently waits for them).

The skill name and plugin are already available as the github.copilot.skill.invoked OTel span and the SDK SKILL_INVOKED event; exposing the same to plugin hooks would cover it.

Example prompts or workflows

User types /my-plugin:my-skill (or /my-skill). A hook configured as "skillInvoked": [{ "type": "command", "command": "...", "matcher": "^my-plugin:" }] runs with { "skillName": "my-plugin:my-skill", "source": "slash_command"}; ordinary prompts run no hook.

Additional context

Copilot CLI 1.0.105, Windows 11. Claude Code already offers this as UserPromptExpansion