Added contribution guidelines and a contribution gate (#1023)

.github/pull_request_template.md

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+<!--
+Thank you for your contribution! Please read our contribution guidelines before you submit:
+https://github.com/MindWorkAI/AI-Studio/blob/main/CONTRIBUTING.md
+
+Please also enable "Allow edits by maintainers" for this pull request. An automated check verifies
+this option together with the required statements below.
+-->
+
+## Summary
+
+<!-- What does this pull request change, and why? -->
+
+## Related proposal or issue
+
+<!-- Link the proposal in our discussions or the issue this pull request is based on. Bug fixes and small improvements may have neither; then write "none". -->
+
+## How did you test it?
+
+<!-- For example: ran `dotnet run verify`, ran the app locally, and tried out … -->
+
+## Required statements
+
+<!-- Please tick each box yourself: these are statements you make personally, see "Licensing of your contribution" in CONTRIBUTING.md. Do not change their wording; the automated check compares it word by word. -->
+
+- [ ] **Review:** I have reviewed every change in this pull request and can explain it when asked.
+- [ ] **License:** I license my contribution in this pull request, including all commits I add to it later, under the MIT License, and I agree that it is distributed as part of MindWork AI Studio under the project license or under the MIT License.
+- [ ] **Right to contribute:** The contribution is my own work, or I have permission to submit it, including the consent of my employer or client where needed. To the best of my knowledge, it does not infringe the rights of others, and AI-generated parts do not reproduce third-party code under incompatible terms.
+- [ ] **Changes by maintainers:** I understand that the maintainers will change this pull request to fit the product, and that they may close it.
+
+## Credits (optional)
+
+<!-- By default, we thank you in the changelog and on the supporters page in the app, with your GitHub username and, if you show it publicly on your GitHub profile, your name. -->
+
+- [ ] Credit me with my GitHub username only.
+- [ ] Do not credit me.