feat(zeta): recipe model, validation, .recipe emitter

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

testdata/golden/mytool.recipe

@@ -0,0 +1,9 @@
+return {
+  name         = "mytool",
+  version      = "1.0",
+  summary      = "Example custom-build package",
+  url          = "mytool-1.0.tar.gz",
+  deps         = {},
+  build_system = "custom",
+  build_script = "build.sh",
+}