refactor(output): share the JSON, created-resource and batch tails

Mutations end with a.jsonResult or a.created instead of hand-rolled
--json branches, the two identical jsonResult/writeMutationJSON helpers
become one, and writeBatchJSON checks --json itself so batch commands
share one tail.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/human_output_test.go

@@ -379,8 +379,12 @@ func TestMutationFeedback(t *testing.T) {
 			ref := refLabel("alice/x", 12, "Fix\x1b[2J login")
 			a.done(cmd, "Created issue %s", ref)
 			a.warn(cmd, "Issue %s is already closed", ref)
-			require.NoError(t, a.printURL(cmd, "https://example.org/12"))
-			require.Equal(t, tc.out, out.String())
+			require.NoError(t, a.created(cmd, map[string]int{"number": 12}, "https://example.org/12"))
+			if tc.json != "" {
+				require.JSONEq(t, `{"number":12}`, out.String())
+			} else {
+				require.Equal(t, tc.out, out.String())
+			}
 			require.Equal(t, tc.stderr, diagnostics.String())
 		})
 	}