Fix 6 Postgres-specific test failures in workflow store

Root causes:
- PG store CreateInstance/CreateAssignment set defaults in local vars
  but didn't write them back to the struct, so callers saw empty
  Status fields (violating CHECK constraints on subsequent updates)
- PG JSONB normalizes whitespace ({"key": "val"} vs {"key":"val"})
  but tests compared exact strings
- ListSignoffs test used "nonexistent" as instance_id but PG
  validates UUID format

Fixes:
- Write defaults directly to inst.Status / a.Status in PG store
  (aligns with SQLite store which already did this)
- Add jsonEq() helper using json.Compact for whitespace-agnostic
  JSON comparison across all stage_data/review_data assertions
- Use valid zero-UUID for non-existent instance in signoff test

All 35 tests pass on SQLite (28 store + 7 engine).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/store/postgres/workflows.go

@@ -406,9 +406,8 @@ func nullIfEmpty(s string) interface{} {
 func (s *WorkflowStore) CreateInstance(ctx context.Context, inst *models.WorkflowInstance) error {
 	stageData := jsonOrEmpty(inst.StageData)
 	metadata := jsonOrEmpty(inst.Metadata)
-	status := inst.Status
-	if status == "" {
-		status = models.InstanceStatusActive
+	if inst.Status == "" {
+		inst.Status = models.InstanceStatusActive
 	}
 	return DB.QueryRowContext(ctx, `
 		INSERT INTO workflow_instances (workflow_id, workflow_version, current_stage,
@@ -416,7 +415,7 @@ func (s *WorkflowStore) CreateInstance(ctx context.Context, inst *models.Workflo
 		VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
 		RETURNING id, stage_entered_at, created_at, updated_at`,
 		inst.WorkflowID, inst.WorkflowVersion, inst.CurrentStage,
-		stageData, status, inst.StartedBy, inst.EntryToken, metadata,
+		stageData, inst.Status, inst.StartedBy, inst.EntryToken, metadata,
 	).Scan(&inst.ID, &inst.StageEnteredAt, &inst.CreatedAt, &inst.UpdatedAt)
 }
 
@@ -575,15 +574,14 @@ func (s *WorkflowStore) ListActiveInstances(ctx context.Context) ([]models.Workf
 
 func (s *WorkflowStore) CreateAssignment(ctx context.Context, a *models.WorkflowAssignment) error {
 	reviewData := jsonOrEmpty(a.ReviewData)
-	status := a.Status
-	if status == "" {
-		status = models.AssignmentStatusUnassigned
+	if a.Status == "" {
+		a.Status = models.AssignmentStatusUnassigned
 	}
 	return DB.QueryRowContext(ctx, `
 		INSERT INTO workflow_assignments (instance_id, stage, team_id, assigned_to, status, review_data)
 		VALUES ($1, $2, $3, $4, $5, $6)
 		RETURNING id, created_at`,
-		a.InstanceID, a.Stage, a.TeamID, a.AssignedTo, status, reviewData,
+		a.InstanceID, a.Stage, a.TeamID, a.AssignedTo, a.Status, reviewData,
 	).Scan(&a.ID, &a.CreatedAt)
 }