Configure split repositories as read-only subtree splits (#3)

- Remove CODEOWNERS files (managed centrally in monorepo)
- Remove ISSUE_TEMPLATE files (issues reported to main repository)
- Remove CI workflows (CI runs in monorepo, not split repos)
- Update PR templates to redirect contributions to monorepo
- Add close-pull-request workflow to auto-close PRs with helpful message
- Fix repository URLs to point to MatesOfMate/.github

Split repositories (common, phpstan-extension, phpunit-extension) are now
read-only mirrors that redirect all contributions to the main monorepo.
This follows the Symfony split repository pattern.

Author: wachterjohannes

.github/CODEOWNERS

@@ -1,20 +1,8 @@
-| Q             | A
-| ------------- | ---
-| Bug fix?      | yes/no
-| New feature?  | yes/no
-| BC breaks?    | yes/no
-| Deprecations? | yes/no
-| Tests pass?   | yes/no
-| Issues        | Fix #... <!-- prefix each issue number with "Fix #" -->
-| License       | MIT
+Please do not submit any Pull Requests here. They will be closed.
+---
 
-<!--
-Replace this notice by a description of your feature/bugfix.
-This will help reviewers and should be a good start for the documentation.
+Please submit your PR here instead:
+https://github.com/MatesOfMate/.github
 
-Please ensure:
- - Tests are added/updated and pass (`composer test`)
- - Code quality checks pass (`composer lint`)
- - Documentation is updated if needed
- - For new tools/resources, provide usage examples
--->
+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/ISSUE_TEMPLATE/1-bug_report.md

@@ -0,0 +1,20 @@
+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/MatesOfMate/.github
+
+            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!