Add workflow to auto-close external PRs (#140)

* Add workflow to auto-close external PRs

* Update PR comment wording

* Add toggle to disable external PR check

.github/workflows/close-external-prs.yml

@@ -0,0 +1,43 @@
+name: Close External PRs
+
+on:
+  pull_request_target:
+    types: [opened]
+
+jobs:
+  check-membership:
+    if: vars.DISABLE_EXTERNAL_PR_CHECK != 'true'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check if author is org member
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const org = 'anthropics';
+            const author = context.payload.pull_request.user.login;
+
+            try {
+              await github.rest.orgs.checkMembershipForUser({
+                org: org,
+                username: author
+              });
+              console.log(`${author} is an org member, allowing PR`);
+            } catch (e) {
+              if (e.status === 404) {
+                await github.rest.issues.createComment({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  issue_number: context.payload.pull_request.number,
+                  body: `Thanks for your interest! This repo only accepts contributions from Anthropic team members. If you'd like to submit a plugin to the marketplace, please submit your plugin [here](https://docs.google.com/forms/d/e/1FAIpQLSdeFthxvjOXUjxg1i3KrOOkEPDJtn71XC-KjmQlxNP63xYydg/viewform).`
+                });
+
+                await github.rest.pulls.update({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  pull_number: context.payload.pull_request.number,
+                  state: 'closed'
+                });
+
+                console.log(`Closed PR from external contributor: ${author}`);
+              }
+            }