Add pkg/go — Go module wrapper for go install distribution

Adds a thin Go wrapper at pkg/go/cmd/codebase-memory-mcp that detects
the current platform and architecture, downloads the matching pre-built
binary from GitHub Releases, verifies its SHA-256 checksum, caches it,
and replaces the current process via exec.

Install via:
  go install github.com/DeusData/codebase-memory-mcp/pkg/go/cmd/codebase-memory-mcp@latest

The Go module proxy indexes this automatically once live on main.