feat(issue/pr edit): support @copilot in assignee flags

- Introduced CopilotReplacer to handle `@copilot` mentions in assignee lists.

pkg/cmd/pr/shared/editable.go

@@ -118,7 +118,28 @@ func (e Editable) AssigneeIds(client *api.Client, repo ghrepo.Interface) (*[]str
 	// curate the final list of assignees from the default list.
 	if len(e.Assignees.Add) != 0 || len(e.Assignees.Remove) != 0 {
 		meReplacer := NewMeReplacer(client, repo.RepoHost())
-		s := set.NewStringSet()
+		copilotReplacer := NewCopilotReplacer()
+
+		// A closure to replace special assignee names with the actual logins.
+		replaceSpecialAssigneeNames := func(value []string) ([]string, error) {
+			replaced, err := meReplacer.ReplaceSlice(value)
+			if err != nil {
+				return nil, err
+			}
+
+			// Only suppported for actor assignees.
+			if e.Assignees.ActorAssignees {
+				replaced, err = copilotReplacer.ReplaceSlice(replaced)
+				if err != nil {
+					return nil, err
+				}
+			}
+
+			return replaced, nil
+		}
+
+		assigneeSet := set.NewStringSet()
+
 		// This check below is required because in a non-interactive flow,
 		// the user gives us a login and not the DisplayName, and when
 		// we have actor assignees e.Assignees.Default will contain
@@ -128,21 +149,24 @@ func (e Editable) AssigneeIds(client *api.Client, repo ghrepo.Interface) (*[]str
 		// Otherwise, the value the user provided won't be found in the
 		// set to be added or removed, causing unexpected behavior.
 		if e.Assignees.ActorAssignees {
-			s.AddValues(e.Assignees.DefaultLogins)
+			assigneeSet.AddValues(e.Assignees.DefaultLogins)
 		} else {
-			s.AddValues(e.Assignees.Default)
+			assigneeSet.AddValues(e.Assignees.Default)
 		}
-		add, err := meReplacer.ReplaceSlice(e.Assignees.Add)
+
+		add, err := replaceSpecialAssigneeNames(e.Assignees.Add)
 		if err != nil {
 			return nil, err
 		}
-		s.AddValues(add)
-		remove, err := meReplacer.ReplaceSlice(e.Assignees.Remove)
+		assigneeSet.AddValues(add)
+
+		remove, err := replaceSpecialAssigneeNames(e.Assignees.Remove)
 		if err != nil {
 			return nil, err
 		}
-		s.RemoveValues(remove)
-		e.Assignees.Value = s.ToSlice()
+		assigneeSet.RemoveValues(remove)
+
+		e.Assignees.Value = assigneeSet.ToSlice()
 	}
 	a, err := e.Metadata.MembersToIDs(e.Assignees.Value)
 	return &a, err

pkg/cmd/pr/shared/params.go

@@ -312,3 +312,30 @@ func (r *MeReplacer) ReplaceSlice(handles []string) ([]string, error) {
 	}
 	return res, nil
 }
+
+// CopilotReplacer resolves usages of `@copilot` to Copilot's login.
+type CopilotReplacer struct{}
+
+func NewCopilotReplacer() *CopilotReplacer {
+	return &CopilotReplacer{}
+}
+
+func (r *CopilotReplacer) replace(handle string) (string, error) {
+	if strings.EqualFold(handle, "@copilot") {
+		return "copilot-swe-agent", nil
+	}
+	return handle, nil
+}
+
+// Replace replaces usages of `@copilot` in a slice with Copilot's login.
+func (r *CopilotReplacer) ReplaceSlice(handles []string) ([]string, error) {
+	res := make([]string, len(handles))
+	for i, h := range handles {
+		var err error
+		res[i], err = r.replace(h)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return res, nil
+}

pkg/cmd/pr/shared/params_test.go

@@ -187,6 +187,59 @@ func TestMeReplacer_Replace(t *testing.T) {
 	}
 }
 
+func TestCopilotReplacer_ReplaceSlice(t *testing.T) {
+	type args struct {
+		handles []string
+	}
+	tests := []struct {
+		name    string
+		args    args
+		want    []string
+		wantErr bool
+	}{
+		{
+			name: "replaces @copilot with copilot-swe-agent",
+			args: args{
+				handles: []string{"monalisa", "@copilot", "hubot"},
+			},
+			want:    []string{"monalisa", "copilot-swe-agent", "hubot"},
+			wantErr: false,
+		},
+		{
+			name: "handles no @copilot mentions",
+			args: args{
+				handles: []string{"monalisa", "user", "hubot"},
+			},
+			want:    []string{"monalisa", "user", "hubot"},
+			wantErr: false,
+		},
+		{
+			name: "replaces multiple @copilot mentions",
+			args: args{
+				handles: []string{"@copilot", "user", "@copilot"},
+			},
+			want:    []string{"copilot-swe-agent", "user", "copilot-swe-agent"},
+			wantErr: false,
+		},
+		{
+			name: "handles @copilot case-insensitively",
+			args: args{
+				handles: []string{"@Copilot", "user", "@CoPiLoT"},
+			},
+			want:    []string{"copilot-swe-agent", "user", "copilot-swe-agent"},
+			wantErr: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			r := NewCopilotReplacer()
+			got, err := r.ReplaceSlice(tt.args.handles)
+			require.NoError(t, err)
+			require.Equal(t, tt.want, got)
+		})
+	}
+}
+
 func Test_QueryHasStateClause(t *testing.T) {
 	tests := []struct {
 		searchQuery string