Document anti-drift usage guidance

[aryeko]

Story

• Shared Contract Kit for Go Variants #69

Motivation

Future changes need guardrails to avoid divergence.

Scope

In:

• Go shared-contract usage guide

Out:

• CI workflow changes

Requirements

• Document expected usage of shared package

Acceptance criteria

• Guide exists and references shared package

Verification

Commands:

• test -f docs/guides/go-shared-contract-kit.md

CI checks:

• (none)

Evidence:

• Guide file

Artifacts

• docs/guides/go-shared-contract-kit.md

Spec reference

• .sisyphus/plans/apps-implementation-roadmap.md
• Section: T-024
• Quote: Document anti-drift usage guidance

Dependencies

Blocked by:

• Integrate shared-contract in baseline/wire/fx/do #77