initial scaffold

- module layout: cmd/mithril-go, internal/{aggregator,artifact,verify,networks}
- aggregator REST client, list command working against mainnet
- download/extract/verify stubbed
- no deps yet, pure stdlib

.gitignore

@@ -0,0 +1,5 @@
+/bin/
+/mithril-go
+*.test
+*.out
+.coverage*