fix: handle Forgejo pull_request_comment payload missing pull_request field

Forgejo's pull_request_comment event uses an issue_comment-like payload
with issue.number instead of pull_request.number. Add fallback to
extract entity number from issue field when pull_request is absent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/github/context.ts

@@ -236,11 +236,21 @@ export function parseGitHubContext(): GitHubContext {
     }
     case "pull_request_review_comment": {
       const payload = context.payload as PullRequestReviewCommentEvent;
+      // Forgejo's pull_request_comment event uses issue_comment-like payload
+      // with issue.number instead of pull_request.number
+      const anyPayload = context.payload as Record<string, unknown>;
+      const issue = anyPayload.issue as { number?: number } | undefined;
+      const entityNumber = payload.pull_request?.number ?? issue?.number;
+      if (!entityNumber) {
+        throw new Error(
+          "Could not determine entity number from pull_request_review_comment event payload",
+        );
+      }
       return {
         ...commonFields,
         eventName: "pull_request_review_comment",
         payload,
-        entityNumber: payload.pull_request.number,
+        entityNumber,
         isPR: true,
       };
     }