docs: establish Go backend repository rules

.gitignore

@@ -0,0 +1,19 @@
+# Raw/local data: keep README only
+/data/incoming/*
+!/data/incoming/README.md
+
+# Environment and secrets
+.env
+.env.*
+!.env.example
+
+# Go outputs and local tooling
+/bin/
+/dist/
+/coverage/
+*.test
+*.out
+/vendor/
+.idea/
+.vscode/
+