stacks: R-687 — an empty leg reports steps [], and a taken files_may_change step names its whole copy

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/stacks/unattended.go

@@ -186,7 +186,7 @@ func (m *Manager) LastUpdateLegSummary() *UpdateLegSummary {
 		return nil
 	}
 	c := *m.leg.last
-	c.Steps = append([]LegStep(nil), m.leg.last.Steps...)
+	c.Steps = append([]LegStep{}, m.leg.last.Steps...) // R-687: `[]`, never `null`, in the hub report
 	return &c
 }
 
@@ -437,6 +437,8 @@ func (m *Manager) legCandidate(ctx context.Context, name string, o *UpdateLegOpt
 			m.logger.Printf("[INFO] [update-leg] %s: the step may change the app's files and no fresh whole copy exists (%s)", name, why)
 			return e, LegSkipFilesNoCopy
 		}
+		// R-687: the TAKEN case names the copy that allowed it, as the skip names why not.
+		m.logger.Printf("[INFO] [update-leg] %s: the step may change the app's files — taken, a fresh whole copy exists: %s", name, why)
 	}
 	return e, ""
 }