docs(go): describe current user defaults and prepare v0.2.1

[muxx]

Go user-selection comments describe legacy envd fallbacks, which are rendered into both public documentation locales. Describe the current template defaults in GoDoc and the Go README, regenerate the owned reference, and keep public documentation free of historical version qualifiers.

The first commit changes documentation only. A separate commit prepares the coordinated v0.2.1 patch release so the documentation repository can pin a corrected immutable release. Runtime behavior and compatibility checks are unchanged.

Addresses the Orpheus finding in documentation MR !21.

Validation:

• Full format, lint, type, and unit checks; Go documentation, vet, build, race, coverage (91.9%), and consumer checks.
• Deterministic generation, release builds, public-artifact validation, and clean artifact-install checks.
• Materialized the corrected Go reference in both documentation locales and verified that the three historical envd qualifiers are absent.
• KVM runtime smoke passed with the v0.2.1 release artifacts: JavaScript core and Code Interpreter, CLI, Python sync/async core and Code Interpreter, and Go template-user inheritance.