Add PR template and update CONTRIBUTING.md with official CLA info

Author: Tarquinen

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,8 @@
+### Summary
+<!-- Provide a brief description of the changes in this PR -->
+
+### Checklist
+- [ ] I have read the [CONTRIBUTING.md](https://github.com/Opencode-DCP/opencode-dynamic-context-pruning/blob/dev/CONTRIBUTING.md)
+- [ ] I have signed the CLA via the CLA Assistant bot (check the bot comment below after opening the PR)
+- [ ] My changes follow the project's coding style
+- [ ] I have added tests to cover my changes (if applicable)