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

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# Contributing
+
+Keep changes focused and preserve Colibri's dependency-free default CPU path.
+
+## Local checks
+
+Run the lightweight checks locally:
+
+```sh
+make -C c check
+```
+
+This performs one portable CPU build, C unit tests, and Python standard-library
+tests. It does not download a model or require CUDA.
+
+CUDA changes should additionally be checked on a CUDA-capable Linux host:
+
+```sh
+make -C c cuda-test CUDA_ARCH=native
+```
+
+Benchmark reports should include the commit, exact commands, hardware and
+storage details, warm-up policy, run count, and median throughput.