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)

cmd/bd/cli_fast_test.go

@@ -333,6 +333,9 @@ func TestCLI_Close(t *testing.T) {
 	if closed[0]["status"] != "closed" {
 		t.Errorf("Expected status 'closed', got: %v", closed[0]["status"])
 	}
+	if closed[0]["close_reason"] != "Done" {
+		t.Errorf("Expected close_reason 'Done', got: %v", closed[0]["close_reason"])
+	}
 }
 
 func TestCLI_DepAdd(t *testing.T) {