Allow validate() to infer description from function name

[masonmark]

Per verbal request from @protiev — when using .validate() with a well-named function, having to also provide a string description is redundant:

// Redundant — the function name already says what it does
script.add('git push').validate(gitIsAvailable, 'git is available');

// Desired — just pass the function, let the name speak for itself
script.add('git push').validate(gitIsAvailable);

Currently, when no description is provided, the output falls back to the generic text "Step validation". Instead, it should use the function's .name property, formatted for display (e.g. gitIsAvailable → "gitIsAvailable").

Current signature

validate(
  check: () => boolean | string | Promise<boolean | string>,
  description?: string,
): this

The signature already supports omitting the description — the improvement is purely in how the fallback is displayed.

Proposed behavior

When description is omitted (or undefined):

1. Use check.name if available (named function or named function expression)
2. Fall back to "Step validation" only for anonymous/arrow functions where .name is empty

In Script.ts line 707, change:

const desc = step.options.validateDescription || 'Step validation';

to something like:

const desc = step.options.validateDescription
  || step.options.validateFn?.name
  || 'Step validation';

This makes the common case of passing a named validation function a one-argument call with useful output.

(Filed from Claude Code VSCode extension, model: Claude Opus 4.6)