🚦 Add workflow for automatic pull requests

This project currently does not accept third-party contributions,
but github does not offer a way to disable this. To work around it,
this adds a workflow that will automatically close PRs that are opened
or reopened, and leaves a comment explaining the rationale for it.

.github/actions/github-close-pr/action.yaml

@@ -0,0 +1,22 @@
+name: github-close-pr
+description: "A tool for automatically closing pull requests"
+
+inputs:
+  message:
+    required: true
+    description: "The message to comment. This may contain "
+
+runs:
+  using: "composite"
+  steps:
+    - name: Leave Issue or Pull Request comment
+      uses: actions/github-script@v7
+      with:
+        retries: 3
+        script: |
+          github.rest.pulls.update({
+            pull_number: context.issue.number,
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            state: 'closed'
+          });

.github/workflows/build.yaml

@@ -2,8 +2,8 @@ name: Build
 on:
   push:
     branches: [master, feature/*]
-  pull_request:
-    branches: [master]
+  # pull_request:
+  #   branches: [master]
   workflow_call:
   workflow_dispatch:
 

.github/workflows/close-pull-requests.yaml

@@ -0,0 +1,40 @@
+name: Close Pull Requests
+on:
+  pull_request_target:
+    types: [opened, reopened]
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Leave comment
+        uses: ./.github/actions/github-comment
+        with:
+          message: |
+            👋🏻 Hi, thank you for your interest in this project.
+
+            I sincerely appreciate your enthusiasm and eagerness to contribute,
+            as public contributions are fundamental to the open-source community.
+
+            However, at this moment, I am **not actively accepting external
+            contributions**. This project primarily exists to experiment and
+            learn about game engine design in Rust -- and as such has a much
+            looser focus and roadmap that is currently aligned with specific
+            goals.
+
+            It is possible that this may change in the future; so please consider
+            starring this project to keep an eye on future updates for
+            opportunities to contribute and collaborate. Your support and
+            understanding are highly valued.
+
+            This project does have
+            [💬 Github Discussion](https://github.com/{{github.repo}}/discussions)
+            enabled if there are suggestions for improvements or reasonable
+            feature requests.
+
+            Thank you! 🙇‍♂️
+
+      - name: Close pull request
+        uses: ./.github/actions/github-close-pr