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.

test/data-formatter.test.ts

@@ -159,6 +159,21 @@ Issue State: OPEN
 Issue Labels: architecture, agent-sdk, drift:functional`,
     );
   });
+
+  test("renders a deleted (null-author) issue author as 'ghost'", () => {
+    const issueData: GitHubIssue = {
+      title: "Test Issue",
+      body: "Issue body",
+      author: null,
+      createdAt: "2023-01-01T00:00:00Z",
+      state: "OPEN",
+      labels: { nodes: [] },
+      comments: { nodes: [] },
+    };
+
+    const result = formatContext(issueData, false);
+    expect(result).toContain("Issue Author: ghost");
+  });
 });
 
 describe("formatBody", () => {
@@ -252,6 +267,24 @@ describe("formatComments", () => {
     );
   });
 
+  test("renders deleted (null-author) comments as 'ghost'", () => {
+    // GitHub returns author: null for comments from deleted accounts.
+    const comments: GitHubComment[] = [
+      {
+        id: "1",
+        databaseId: "100001",
+        body: "From a deleted account",
+        author: null,
+        createdAt: "2023-01-01T00:00:00Z",
+      },
+    ];
+
+    const result = formatComments(comments);
+    expect(result).toBe(
+      "[ghost at 2023-01-01T00:00:00Z]: From a deleted account",
+    );
+  });
+
   test("returns empty string for empty comments array", () => {
     const result = formatComments([]);
     expect(result).toBe("");
@@ -494,6 +527,29 @@ describe("formatReviewComments", () => {
     );
   });
 
+  test("renders deleted (null-author) reviews as 'ghost'", () => {
+    const reviewData = {
+      nodes: [
+        {
+          id: "review1",
+          databaseId: "300099",
+          author: null,
+          body: "Left before deleting the account",
+          state: "COMMENTED",
+          submittedAt: "2023-01-01T00:00:00Z",
+          comments: {
+            nodes: [],
+          },
+        },
+      ],
+    };
+
+    const result = formatReviewComments(reviewData);
+    expect(result).toBe(
+      `[Review by ghost at 2023-01-01T00:00:00Z]: COMMENTED\nLeft before deleting the account`,
+    );
+  });
+
   test("formats multiple reviews correctly", () => {
     const reviewData = {
       nodes: [