docs: add Go SDK reference contract

[muxx]

Summary

• generate deterministic Go API reference for the core SDK and Code Interpreter with pinned gomarkdoc
• enforce documentation for exported handwritten Go declarations
• publish Go reference metadata and versioned pkg.go.dev links
• extend CI and release verification for GoDoc and pkg.go.dev availability

Verification

• make go-check
• make generate (twice; second run produced no diff)
• pnpm format:check
• pnpm lint
• pnpm typecheck
• pnpm test
• ./scripts/build-release.sh
• node scripts/check-public-artifacts.mjs release
• ./scripts/test-release-artifacts.sh