Allow skills to be activated manually like /commands

### What would you like to be added?

The ability to manually trigger skill activation, e.g. by typing `/skill-name`.

### Why is this needed?

Occasionally, I'll work on a task for which I know an installed skill is relevant, but the agent won't reliably activate the skill in response to my prompt (despite my best efforts to prompt engineer the skill description).

Caveat: I can usually get the skill to activate by prompting the agent with "activate the `$SKILLNAME` skill", but the command completion menu (typing `/` and seeing the list of available commands and their descriptions) would be handy to extend to include skills as well.

### Additional context

According to their docs, this functionality is supported by [Claude Code](https://code.claude.com/docs/en/skills).