feat: add negation support for step conditions (!{{var}})

Adds "!{{var}}" syntax for negated truthy checks in Step.Condition.
Useful for "skip this step if feature is enabled" patterns.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/formula/stepcondition.go

@@ -5,6 +5,7 @@
 //
 // Supported formats:
 //   - "{{var}}" - truthy check (non-empty, non-"false", non-"0")
+//   - "!{{var}}" - negated truthy check (include if var is falsy)
 //   - "{{var}} == value" - equality check
 //   - "{{var}} != value" - inequality check
 package formula
@@ -20,6 +21,9 @@ var (
 	// {{var}} - simple variable reference for truthy check
 	stepCondVarPattern = regexp.MustCompile(`^\{\{(\w+)\}\}$`)
 
+	// !{{var}} - negated truthy check
+	stepCondNegatedVarPattern = regexp.MustCompile(`^!\{\{(\w+)\}\}$`)
+
 	// {{var}} == value or {{var}} != value
 	stepCondComparePattern = regexp.MustCompile(`^\{\{(\w+)\}\}\s*(==|!=)\s*(.+)$`)
 )
@@ -30,6 +34,7 @@ var (
 // Condition formats:
 //   - "" (empty) - always include
 //   - "{{var}}" - include if var is truthy (non-empty, non-"false", non-"0")
+//   - "!{{var}}" - include if var is NOT truthy (negated)
 //   - "{{var}} == value" - include if var equals value
 //   - "{{var}} != value" - include if var does not equal value
 func EvaluateStepCondition(condition string, vars map[string]string) (bool, error) {
@@ -47,6 +52,13 @@ func EvaluateStepCondition(condition string, vars map[string]string) (bool, erro
 		return isTruthy(value), nil
 	}
 
+	// Try negated truthy pattern: !{{var}}
+	if m := stepCondNegatedVarPattern.FindStringSubmatch(condition); m != nil {
+		varName := m[1]
+		value := vars[varName]
+		return !isTruthy(value), nil
+	}
+
 	// Try comparison pattern: {{var}} == value or {{var}} != value
 	if m := stepCondComparePattern.FindStringSubmatch(condition); m != nil {
 		varName := m[1]