Function slugify_string 

pub fn slugify_string(input: &str) -> String

String-level slug helper used by slugify_for_palace.

Why: exposed separately so the CLI can slugify an arbitrary repo name (e.g. from --to my_project) without re-deriving from a path. What: applies the canonicalisation rules described on slugify_for_palace. Test: tests::slug_derivation_cases.