Local project checks and contribution templates: make check, dependency-free C/python tests, issue forms + blank issues, CONTRIBUTING (#20)

* Add lightweight project checks and templates

* Ignore generated test binaries

* Keep project checks local

.github/pull_request_template.md

@@ -0,0 +1,14 @@
+## Summary
+
+Describe the problem and the smallest change that solves it.
+
+## Validation
+
+- [ ] `make -C c check`
+- [ ] CUDA changes were tested with `make -C c cuda-test` (if applicable)
+- [ ] Performance claims include hardware, commands, and repeatable measurements
+
+## Compatibility
+
+- [ ] The default CPU build remains dependency-free
+- [ ] No model files, generated binaries, or benchmark artifacts are included