Propose missing facets for an imported skill

Propose taxonomy, keywords, tags, and connected integrations that the derived manifest does not already declare. Every field is a suggestion that a person confirms or edits before saving. This single endpoint serves pasted and GitHub imports because both now supply the same derived manifest and prose. It stays separate from the local preview because a person may re-run that preview after each edit, while this proposal makes a metered model call. A GitHub scan previews every candidate, so combining the calls would spend once per candidate before the person chooses one. A proposal timeout also returns an empty suggestion without removing a valid preview from the screen.