fix(sdlc): make phase transitions idempotent

Allow transitioning to the current phase (no-op success) instead of
rejecting it as a "backward" transition. This fixes issues where
external systems retry transition commands.

Before: draft -> draft returned error
After: draft -> draft returns nil (already there)

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

internal/sdlc/feature.go

@@ -140,7 +140,13 @@ func (f *Feature) CanTransitionTo(target FeaturePhase, cfg *Config) error {
 	currentIdx := PhaseIndex(f.Phase)
 	targetIdx := PhaseIndex(target)
 
-	if targetIdx <= currentIdx {
+	// Idempotent: if already in target phase, return success
+	if targetIdx == currentIdx {
+		return nil
+	}
+
+	// Reject backward transitions
+	if targetIdx < currentIdx {
 		return fmt.Errorf("%w: cannot move from %s to %s (backward)", ErrInvalidTransition, f.Phase, target)
 	}
 

internal/sdlc/feature_test.go

@@ -128,9 +128,9 @@ func TestCanTransitionTo(t *testing.T) {
 		t.Error("CanTransitionTo(planned) = nil, want error (skip)")
 	}
 
-	// Invalid: draft -> draft (backward)
-	if err := f.CanTransitionTo(PhaseDraft, cfg); err == nil {
-		t.Error("CanTransitionTo(draft) = nil, want error (backward)")
+	// Idempotent: draft -> draft (allowed, returns nil)
+	if err := f.CanTransitionTo(PhaseDraft, cfg); err != nil {
+		t.Errorf("CanTransitionTo(draft) = %v, want nil (idempotent)", err)
 	}
 
 	// Invalid phase