fix(storage): persist close_reason to issues table on close (#551)

CloseIssue was storing the reason only in the events table, not in the
issues.close_reason column. This caused `bd show --json` to return an
empty close_reason even when one was provided.

- Update CloseIssue in queries.go and transaction.go to set close_reason
- Clear close_reason when reopening issues (in manageClosedAt)
- Add tests for close_reason in storage and CLI JSON output
- Document the dual-storage of close_reason (issues + events tables)

internal/storage/sqlite/transaction_test.go

@@ -268,6 +268,9 @@ func TestTransactionCloseIssue(t *testing.T) {
 	if closed.Status != types.StatusClosed {
 		t.Errorf("expected status 'closed', got %q", closed.Status)
 	}
+	if closed.CloseReason != "Done" {
+		t.Errorf("expected close_reason 'Done', got %q", closed.CloseReason)
+	}
 }
 
 // TestTransactionDeleteIssue tests deleting an issue within a transaction.