Fix expansion operators: update dependencies after expansion

When a step is expanded (e.g., implement -> implement.draft, implement.refine-1,
etc.), any steps that depended on the original step should now depend on the
last step of the expansion.

Call UpdateDependenciesForExpansion after both expand and map operations.

Fixes gt-8tmz.3

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

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

internal/formula/expand.go

@@ -82,6 +82,13 @@ func ApplyExpansions(steps []*Step, compose *ComposeRules, parser *Parser) ([]*S
 		result = replaceStep(result, rule.Target, expandedSteps)
 		expanded[rule.Target] = true
 
+		// Update dependencies: any step that depended on the target should now
+		// depend on the last step of the expansion
+		if len(expandedSteps) > 0 {
+			lastStepID := expandedSteps[len(expandedSteps)-1].ID
+			result = UpdateDependenciesForExpansion(result, rule.Target, lastStepID)
+		}
+
 		// Update step map with new steps
 		for _, s := range expandedSteps {
 			stepMap[s.ID] = s
@@ -122,6 +129,13 @@ func ApplyExpansions(steps []*Step, compose *ComposeRules, parser *Parser) ([]*S
 			result = replaceStep(result, targetStep.ID, expandedSteps)
 			expanded[targetStep.ID] = true
 
+			// Update dependencies: any step that depended on the target should now
+			// depend on the last step of the expansion
+			if len(expandedSteps) > 0 {
+				lastStepID := expandedSteps[len(expandedSteps)-1].ID
+				result = UpdateDependenciesForExpansion(result, targetStep.ID, lastStepID)
+			}
+
 			// Update step map
 			for _, s := range expandedSteps {
 				stepMap[s.ID] = s