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>

internal/storage/sqlite/sqlite.go

@@ -1170,8 +1170,14 @@ func manageClosedAt(oldIssue *types.Issue, updates map[string]interface{}, setCl
 		return setClauses, args
 	}
 
-	newStatus, ok := statusVal.(string)
-	if !ok {
+	// Handle both string and types.Status
+	var newStatus string
+	switch v := statusVal.(type) {
+	case string:
+		newStatus = v
+	case types.Status:
+		newStatus = string(v)
+	default:
 		return setClauses, args
 	}