Make an index skill

We keep having the same problems:
- where do we put the skills that we maintain?
- how do we compose skills provided by others? (must be updatable)
- how do we deliver those skills?

Rather than building tooling, I propose we build an index, with a wrapper skill

`/find-skill to help me do some NI analysis STAMPED with datalad`

```md
skills-index.md

name: some-con-skill
url: gist.github.com/con/skills/some-con-skill
when-to-use: when doing something for con skillfully
detailed-description: Does this this this this this and that
```

The find-skill should not load a huge thing, it should look over the "when-to-use" find possible hits, read the detailed description, and then install the skill. It should be able to pull raw-text from url, and install the skill. Option to use "oneoff" or to install the skill into the project using datalad download, and able to update that skill from the same source.

Start with the proposed skills-index.md format and the /find-skill entry point, then review how skills are currently maintained and delivered. Define completion around finding skills from their when-to-use and detailed-description fields, fetching them from a URL, supporting one-off use or project installation, and updating an installed skill from the same source.

Written by the indexing model from the issue text.