HasPrefix, HasSuffix, TrimPrefix, TrimSuffix, Contains, Split, SplitN,
StringJoin, Replace, Lower, Upper, Trim, RuneCount.
utils.go and command.go now use string.go helpers — zero direct
strings import in either file.
234 tests, 79.8% coverage.
Co-Authored-By: Virgil <virgil@lethean.io>