Create a migration and compatibility policy for public SDK APIs

[David-patrick-chuks]

Summary

Define the project's stance on semantic versioning, deprecation, and compatibility expectations for external users.

Why this matters

A contributor-friendly SDK needs a clear public contract. Without that, roadmap changes can undermine trust and make releases harder to manage.

Scope

• Document compatibility expectations for public APIs.
• Define how breaking changes and deprecations are communicated.
• Align policy with planned versioned API architecture.
• Keep the policy practical for a growing SDK.

Acceptance Criteria

• A compatibility policy document exists in the repo.
• Versioning and deprecation expectations are clear.
• Policy aligns with current packaging and API architecture direction.
• Maintainers can reference it in future issue and release work.

Dependencies

• Depends on Introduce versioned API route resolution for future client evolution #2