ci: add Default MR template

.gitlab/merge_request_templates/Default.md

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+## What does this MR do?
+
+<!-- Brief description of the change -->
+
+## Motivation
+
+<!-- Why is this change needed? Link related issues: Closes #123 -->
+
+## Type of change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Breaking change
+- [ ] Documentation
+- [ ] CI/CD / Infrastructure
+- [ ] Refactor
+
+## Testing
+
+- [ ] Existing tests pass (`cargo test --workspace --locked`)
+- [ ] New tests added for new functionality
+- [ ] Manually tested on dev node
+
+## Checklist
+
+- [ ] Code follows project style (`cargo fmt`, `cargo clippy`)
+- [ ] CHANGELOG.md updated (for user-facing changes)
+- [ ] Documentation updated if needed
+- [ ] No `FAIL-CI` markers left in code
+- [ ] Breaking changes noted
+
+/assign me
+