fix: handle null comment/review author from deleted accounts (#1490)

GitHub's GraphQL author field is null when the account behind a
comment, review, PR, or issue has been deleted (the ghost user). The
action typed author as non-null and read author.login directly, so a
single comment from a deleted account threw and was swallowed into a
generic 'Failed to fetch PR/issue data', failing the entire run.

Make author nullable on the four affected types and fall back to
'ghost' at each login read. With the type nullable, tsc flags every
dereference, so all sites are covered.

src/github/types.ts

@@ -1,4 +1,8 @@
 // Types for GitHub GraphQL query responses
+
+// GitHub's GraphQL `author`/`actor` fields resolve to null when the underlying
+// account has been deleted (the "ghost" user). Any field typed as
+// `GitHubAuthor | null` can therefore be null at runtime and must be guarded.
 export type GitHubAuthor = {
   login: string;
   name?: string;
@@ -8,7 +12,7 @@ export type GitHubComment = {
   id: string;
   databaseId: string;
   body: string;
-  author: GitHubAuthor;
+  author: GitHubAuthor | null;
   createdAt: string;
   updatedAt?: string;
   lastEditedAt?: string;
@@ -39,7 +43,7 @@ export type GitHubFile = {
 export type GitHubReview = {
   id: string;
   databaseId: string;
-  author: GitHubAuthor;
+  author: GitHubAuthor | null;
   body: string;
   state: string;
   submittedAt: string;
@@ -53,7 +57,7 @@ export type GitHubReview = {
 export type GitHubPullRequest = {
   title: string;
   body: string;
-  author: GitHubAuthor;
+  author: GitHubAuthor | null;
   baseRefName: string;
   headRefName: string;
   headRefOid: string;
@@ -95,7 +99,7 @@ export type GitHubPullRequest = {
 export type GitHubIssue = {
   title: string;
   body: string;
-  author: GitHubAuthor;
+  author: GitHubAuthor | null;
   createdAt: string;
   updatedAt?: string;
   lastEditedAt?: string;