Fix bd-206: Handle status transitions and closed_at constraint

- Updated manageClosedAt to handle both string and types.Status type assertions
- Added equalTime function for comparing timestamps in import change detection
- Added tests for open→closed and closed→open transitions
- Added comment clarifying closed_at is managed automatically

The bug occurred when UpdateIssue received types.Status instead of string,
causing manageClosedAt to skip setting closed_at when status changed to closed.

Amp-Thread-ID: https://ampcode.com/threads/T-ee774f6d-3b90-4311-976d-60c8dd8fe677
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/import_phases.go

@@ -181,16 +181,19 @@ func upsertIssues(ctx context.Context, sqliteStore *sqlite.SQLiteStorage, issues
 			updates["acceptance_criteria"] = issue.AcceptanceCriteria
 			updates["notes"] = issue.Notes
 
+			// bd-206: closed_at is managed automatically by UpdateIssue based on status
+			// No need to set it explicitly here
+
 			if issue.Assignee != "" {
-				updates["assignee"] = issue.Assignee
+			updates["assignee"] = issue.Assignee
 			} else {
-				updates["assignee"] = nil
+			updates["assignee"] = nil
 			}
 
 			if issue.ExternalRef != nil && *issue.ExternalRef != "" {
-				updates["external_ref"] = *issue.ExternalRef
+			updates["external_ref"] = *issue.ExternalRef
 			} else {
-				updates["external_ref"] = nil
+			updates["external_ref"] = nil
 			}
 
 			// bd-88: Only update if data actually changed (prevents timestamp churn)