add justfile and contributing docs

CONTRIBUTING.md

···
+# contributing
+
+## before committing
+
+```sh
+just fmt
+```
+
+or without just:
+
+```sh
+zig fmt .
+```
+
+CI runs `zig fmt --check .` and will fail on unformatted code.

justfile

···
+# zat
+
+# show available commands
+default:
+    @just --list
+
+# format code
+fmt:
+    zig fmt .
+
+# check formatting (CI)
+check:
+    zig fmt --check .
+
+# run tests
+test:
+    zig build test