refactor(recipe): improve subRe regexp and add explanatory comments

Simplify the subRe regular expression by using a non-capturing group for the section labels instead of alternation, and switch the lookahead to a non-capturing group for consistency. Add comments to document the purpose of stepRe and subRe for better maintainability.

internal/recipe/loader.go

@@ -12,8 +12,12 @@ import (
 )
 
 var (
+	// stepRe finds every "### Step N: Title" heading
 	stepRe = regexp.MustCompile(`(?m)^### Step (\d+): (.+)$`)
-	subRe  = regexp.MustCompile(`(?m)^(\*\*Objective:\*\*|\*\*Instructions:\*\*|\*\*Expected output:\*\*)\s*(.+?)(?=\n\n|\n###|\z)`)
+
+	// subRe finds the three labelled sections inside each step.
+	// We use a simple non-capturing group + word-boundary approach instead of lookahead.
+	subRe = regexp.MustCompile(`(?m)^(\*\*(?:Objective|Instructions|Expected output):\*\*)\s*(.+?)(?:\n\n|\n###|\z)`)
 )
 
 // Load reads a recipe from disk and fully parses it.