Add --remove-type flag to gh issue edit

Pair --type with --remove-type so callers can clear an issue's type
without going through the interactive editor, mirroring the
--milestone / --remove-milestone and --parent / --remove-parent
patterns. The two type flags are mutually exclusive.

UpdateIssueIssueType now sends a null issueTypeId when the caller
passes an empty string, which is what the API requires to clear the
field. The orchestrator fires the mutation when either IssueTypeID is
non-empty or RemoveIssueType is set.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

pkg/cmd/issue/edit/edit_test.go

@@ -297,6 +297,19 @@ func TestNewCmdEdit(t *testing.T) {
 				},
 			},
 		},
+		{
+			name:  "remove-type flag",
+			input: "23 --remove-type",
+			output: EditOptions{
+				IssueNumbers:    []int{23},
+				RemoveIssueType: true,
+			},
+		},
+		{
+			name:     "both type and remove-type flags",
+			input:    "23 --type Bug --remove-type",
+			wantsErr: true,
+		},
 		{
 			name:  "parent flag",
 			input: "23 --parent 100",
@@ -855,6 +868,29 @@ func Test_editRun(t *testing.T) {
 			},
 			stdout: "https://github.com/OWNER/REPO/issue/123\n",
 		},
+		{
+			name: "remove type",
+			input: &EditOptions{
+				Detector:        &fd.EnabledDetectorMock{},
+				IssueNumbers:    []int{123},
+				Interactive:     false,
+				RemoveIssueType: true,
+				FetchOptions:    prShared.FetchOptions,
+			},
+			httpStubs: func(t *testing.T, reg *httpmock.Registry) {
+				mockIssueGet(t, reg)
+				reg.Register(
+					httpmock.GraphQL(`mutation UpdateIssueIssueType\b`),
+					httpmock.GraphQLMutation(`
+					{ "data": { "updateIssueIssueType": { "issue": { "id": "123" } } } }`,
+						func(inputs map[string]interface{}) {
+							assert.Equal(t, "123", inputs["issueId"])
+							assert.Nil(t, inputs["issueTypeId"])
+						}),
+				)
+			},
+			stdout: "https://github.com/OWNER/REPO/issue/123\n",
+		},
 		{
 			name: "interactive edit type prompt",
 			input: &EditOptions{