Add PR template and auto-close PR on subtree split repositories

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,8 @@
+Please do not submit any Pull Requests here. They will be closed.
+---
+
+Please submit your PR here instead:
+https://github.com/php-flasher/php-flasher
+
+This repository is what we call a "subtree split": a read-only subset of that main repository.
+We're looking forward to your PR there!

.github/workflows/close-pull-request.yml

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+name: Close Pull Request
+
+on:
+    pull_request_target:
+        types: [opened]
+
+jobs:
+    run:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: superbrothers/close-pull-request@v3
+              with:
+                  comment: |
+                      Thanks for your Pull Request! We love contributions.
+
+                      However, you should instead open your PR on the main repository:
+                      https://github.com/php-flasher/php-flasher
+
+                      This repository is what we call a "subtree split": a read-only subset of that main repository.
+                      We're looking forward to your PR there!