feat: add desc field to required variables

[JakesMD]

What changed

• Added an optional desc field for required variables. The description is shown in the error message when the variable is missing or in the interactive prompt.

    requires:
        vars:
          - name: VERSION
            desc: Semantic version to release, e.g. 1.4.2

  task: Task "release" cancelled because it is missing required variables:
  VERSION (Semantic version to release, e.g. 1.4.2)

  ? Enter value for VERSION (Semantic version to release, e.g. 1.4.2):

• Added tests
• Updated schema.md
• Added a dedicated section to guide.md

Use cases

• clear up any ambiguity for what the variable is for
• provide example values
• provide instructions on where to find its value
• provide a format for the value, e.g. dd/mm/yy

Disclosure

• AI was used to implement changes
• I have read and followed the Contribution Guide