Replace @copilot with Copilot reviewer login in gh pr create

Wire CopilotReviewerReplacer into NewIssueState so that
`gh pr create --reviewer @copilot` correctly resolves to the
copilot-pull-request-reviewer bot login, matching the behavior
already implemented in gh pr edit.

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

pkg/cmd/pr/create/create.go

@@ -680,9 +680,12 @@ func NewIssueState(ctx CreateContext, opts CreateOptions) (*shared.IssueMetadata
 		return nil, err
 	}
 
+	copilotReplacer := shared.NewCopilotReviewerReplacer()
+	reviewers := copilotReplacer.ReplaceSlice(opts.Reviewers)
+
 	state := &shared.IssueMetadataState{
 		Type:          shared.PRMetadata,
-		Reviewers:     opts.Reviewers,
+		Reviewers:     reviewers,
 		Assignees:     assignees,
 		Labels:        opts.Labels,
 		ProjectTitles: opts.Projects,

pkg/cmd/pr/create/create_test.go

@@ -1523,6 +1523,42 @@ func Test_createRun(t *testing.T) {
 			expectedOut:    "https://github.com/OWNER/REPO/pull/12\n",
 			expectedErrOut: "",
 		},
+		{
+			name: "@copilot reviewer resolves to bot login",
+			setup: func(opts *CreateOptions, t *testing.T) func() {
+				opts.TitleProvided = true
+				opts.BodyProvided = true
+				opts.Title = "my title"
+				opts.Body = "my body"
+				opts.Reviewers = []string{"hubot", "@copilot"}
+				opts.HeadBranch = "feature"
+				return func() {}
+			},
+			httpStubs: func(reg *httpmock.Registry, t *testing.T) {
+				reg.Register(
+					httpmock.GraphQL(`mutation PullRequestCreate\b`),
+					httpmock.GraphQLMutation(`
+						{ "data": { "createPullRequest": { "pullRequest": {
+							"URL": "https://github.com/OWNER/REPO/pull/12",
+							"id": "NEWPULLID"
+						} } } }`,
+						func(input map[string]interface{}) {}))
+				reg.Register(
+					httpmock.GraphQL(`mutation RequestReviewsByLogin\b`),
+					httpmock.GraphQLMutation(`
+					{ "data": { "requestReviewsByLogin": {
+						"clientMutationId": ""
+					} } }
+				`, func(inputs map[string]interface{}) {
+						assert.Equal(t, "NEWPULLID", inputs["pullRequestId"])
+						assert.Equal(t, []interface{}{"hubot"}, inputs["userLogins"])
+						assert.Equal(t, []interface{}{"copilot-pull-request-reviewer[bot]"}, inputs["botLogins"])
+						assert.Equal(t, true, inputs["union"])
+					}))
+			},
+			expectedOut:    "https://github.com/OWNER/REPO/pull/12\n",
+			expectedErrOut: "",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {