Step 16: Polish — docs, flake, goreleaser, e2e test.
Phase 2 complete.
ARCHITECTURE.md: full rewrite covering gRPC protocol, SSH auth,
updated package structure, all Garden methods, design decisions.
README.md: add remote sync section, mirror/prune commands, sgardd usage.
CLAUDE.md: add gRPC/proto/x-crypto deps, server/client/sgardpb packages.
flake.nix: build both sgard + sgardd, updated vendorHash.
goreleaser: add sgardd build target.
E2e test: full push/pull cycle with SSH auth between two clients.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>