fix: handle null files field from GraphQL on very large PRs (#1593)

GitHub's GraphQL API returns files: null (with no errors entry, and
changedFiles misreported as 0) when a PR's diff is too large to compute.
The unguarded pullRequest.files.nodes dereference in the fetcher crashed
the action with 'TypeError: null is not an object', and the formatter had
the same latent crash on prData.files.nodes.length.

Widen the GitHubPullRequest type to files | null so the compiler enforces
guards, degrade gracefully in the fetcher with a warning, and render the
file count as unavailable (not '0 files') in the formatter.

Fixes #1587

src/github/types.ts

@@ -85,9 +85,13 @@ export type GitHubPullRequest = {
       commit: GitHubCommit;
     }>;
   };
+  // GitHub's GraphQL `files` field resolves to null when the PR's diff is too
+  // large for GitHub to compute (very large PRs). `changedFiles` is also
+  // misreported as 0 in that case, so the null must be guarded and treated as
+  // "file list unavailable" rather than "no files changed".
   files: {
     nodes: GitHubFile[];
-  };
+  } | null;
   comments: {
     nodes: GitHubComment[];
   };