Skip comments on issues

The bot is currently triggered for comments on both issues and pull
requests, but the bot has no purpose on issues. If the job does end
up getting run, it will fail with an error.

The example in the README has been updated to skip the job when
triggered by issue comments. The runner will also bail early if it
determines that it's being run for an issue comment rather than a pull
request comment, instead of blowing up when it tries to get the pull
request authors.

Fixes Roblox#10

README.md

@@ -31,6 +31,8 @@ on:
 
 jobs:
   CLABot:
+    # Skip job when triggered by an issue comment rather than a PR comment
+    if: github.event_name == 'pull_request_target' || contains(github.event.comment.html_url, '/pull/')
     runs-on: ubuntu-latest
     steps:
       - name: "CLA Signature Bot"

src/claRunner.ts

@@ -1,4 +1,5 @@
 import * as core from '@actions/core';
+import { context } from "@actions/github";
 import { Author } from "./authorMap";
 import { BlockchainPoster } from "./blockchainPoster";
 import { ClaFileRepository } from "./claFileRepository";
@@ -47,6 +48,9 @@ export class ClaRunner {
             // PR is closed and should be locked to preserve signatures.
             await this.lockPullRequest();
             return true;
+        } else if (!context.payload.issue?.pull_request) {
+            core.info("Skipping issue comment")
+            return true
         }
 
         // Just drop allowlisted authors entirely, no sense in processing them.