Variable SLUG_PATTERN`Const`

SLUG_PATTERN: "^[a-z0-9][a-z0-9-]*$" = '^[a-z0-9][a-z0-9-]*$'

Branch- and path-safe slug grammar as a regex source string (the ID_PATTERN precedent) — carriers (CLI, web form) ship it to clients as data instead of re-deriving it.