Use a typed command model in StackRepl execution API

[ForNeVeR]

Problem

StackRepl.execute accepts commands as raw strings and has a TODO to switch to a typed GhciCommand representation.

Why it matters

Stringly-typed commands are error-prone and make command construction and validation harder to maintain.

Affected code locations

• haskeletor/src/main/scala/me/fornever/haskeletor/external/repl/StackRepl.scala

  Line 92 in d71af7e

  // TODO: command should be a GhciCommand instead of a String