Add Go SDK with CI and release support

[muxx]

Summary

• add the public Go 1.24+ SDK for AgentBox sandboxes, commands, PTY, filesystem, templates, snapshots, IAM, networking, and Code Interpreter
• generate internal OpenAPI, protobuf, and Connect clients from checked-in specifications and document parity and Go-version maintenance
• add hermetic tests, race and coverage gates, external-consumer checks, deterministic generation, the Go 1.24-1.27 CI matrix, cross-builds, and submodule-tag release verification
• harden streaming behavior so command output is lossless, unused output pumps are reclaimed, uploads have explicit streaming timeouts, and Connect response streams are always closed

Verification

• pnpm install --frozen-lockfile
• uv sync --project packages/python-sdk --frozen
• uv sync --project packages/code-interpreter-python --frozen
• pnpm format:check
• pnpm lint
• pnpm typecheck
• pnpm test
• make go-check
• Go build and test on 1.24.13, 1.25.14, 1.26.8, and 1.27.1
• Linux, macOS, and Windows cross-builds on Go 1.24 and 1.27
• make generate && git diff --exit-code
• ./scripts/build-release.sh
• node scripts/check-public-artifacts.mjs release
• ./scripts/test-release-artifacts.sh
• actionlint

Release note

packages/go-sdk/version.go intentionally remains at 0.1.1. The coordinated 0.1.2 version bump for all SDK packages will be handled separately.