refactor(pr shared): consolidate ActorAssignees and ActorReviewers into ApiActorsSupported

The CLI had two per-entity flags (ActorAssignees on EditableAssignees and
IssueMetadataState, ActorReviewers on IssueMetadataState) threaded through
different layers of the stack to distinguish github.com from GHES. Both
flags were always set from the same source (issueFeatures.ActorIsAssignable)
and never had different values, but they were carried independently on
different structs. This led to a confusing asymmetry where:

- EditableAssignees had ActorAssignees but EditableReviewers had nothing
- The PR edit flow piggybacked on editable.Assignees.ActorAssignees to
  make reviewer mutation decisions, which was misleading
- RepoMetadataInput only had ActorAssignees with no reviewer equivalent

This commit replaces all per-entity flags with a single ApiActorsSupported
bool hoisted to the shared level on Editable, IssueMetadataState, and
RepoMetadataInput. Both assignees and reviewers now key off the same signal.

Every branch site is marked with // TODO ApiActorsSupported so we can grep
for cleanup sites when GHES eventually supports the actor-based mutations
(replaceActorsForAssignable, requestReviewsByLogin).

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

api/queries_issue.go

@@ -311,7 +311,7 @@ func IssueCreate(client *Client, repo *Repository, params map[string]interface{}
 	}
 	issue := &result.CreateIssue.Issue
 
-	// Assign users using login-based mutation when ActorAssignees is true (github.com).
+	// Assign users using login-based mutation when ApiActorsSupported is true (github.com).
 	if assigneeLogins, ok := params["assigneeLogins"].([]string); ok && len(assigneeLogins) > 0 {
 		err := ReplaceActorsForAssignableByLogin(client, repo, issue.ID, assigneeLogins)
 		if err != nil {

api/queries_pr.go

@@ -524,7 +524,7 @@ func CreatePullRequest(client *Client, repo *Repository, params map[string]inter
 		}
 	}
 
-	// Assign users using login-based mutation when ActorAssignees is true (github.com).
+	// Assign users using login-based mutation when ApiActorsSupported is true (github.com).
 	if assigneeLogins, ok := params["assigneeLogins"].([]string); ok && len(assigneeLogins) > 0 {
 		err := ReplaceActorsForAssignableByLogin(client, repo, pr.ID, assigneeLogins)
 		if err != nil {

api/queries_repo.go

@@ -922,14 +922,15 @@ func (m *RepoMetadataResult) Merge(m2 *RepoMetadataResult) {
 }
 
 type RepoMetadataInput struct {
-	Assignees      bool
-	ActorAssignees bool
-	Reviewers      bool
-	TeamReviewers  bool
-	Labels         bool
-	ProjectsV1     bool
-	ProjectsV2     bool
-	Milestones     bool
+	Assignees     bool
+	Reviewers     bool
+	TeamReviewers bool
+	// TODO ApiActorsSupported
+	ApiActorsSupported bool
+	Labels             bool
+	ProjectsV1         bool
+	ProjectsV2         bool
+	Milestones         bool
 }
 
 // RepoMetadata pre-fetches the metadata for attaching to issues and pull requests
@@ -938,7 +939,8 @@ func RepoMetadata(client *Client, repo ghrepo.Interface, input RepoMetadataInput
 	var g errgroup.Group
 
 	if input.Assignees || input.Reviewers {
-		if input.ActorAssignees {
+		// TODO ApiActorsSupported
+		if input.ApiActorsSupported {
 			g.Go(func() error {
 				actors, err := RepoAssignableActors(client, repo)
 				if err != nil {