Stabilize public API, SDK, and extension contracts for `0.1.0`

Summary

Before the OpenShell 0.1.0 release, review and stabilize every public API, SDK, and extension contract. This is the final planned opportunity to make coordinated breaking changes before those surfaces are treated as stable as defined in RFC-0014.

We'll use this ticket to track specific API changes as child tickets.

Scope

Contract inventory and review

Create an inventory of all externally consumed contracts and identify an owner for each surface, including:

• Gateway gRPC services, messages, methods, status/error behavior, and transport metadata
• SDK APIs and generated client types across supported languages
• CLI- or configuration-facing representations derived from public API types
• Extension contracts, including gateway interceptors, supervisor middleware, compute drivers, credential drivers, and other supported extension points
• Serialization formats, enum behavior, identifiers, pagination, optionality/defaults, version negotiation, and capability discovery

For every contract, review naming, structure, semantics, consistency, extensibility, error handling, and compatibility risks. Explicitly classify each surface as public/stable, public/experimental, or internal.

Final pre-beta breaking-change pass

• Resolve the contract changes identified by the review as one coordinated pre-beta stabilization effort.
• Remove or replace APIs that we do not intend to support during beta.
• Align equivalent concepts and behaviors across gRPC, SDKs, CLI/configuration, and extension surfaces.
• Regenerate affected clients and fixtures and update all in-repository consumers.
• Publish migration notes that enumerate every breaking change and show consumers how to update.
• Establish a cutoff after which public beta contracts follow the compatibility policy below.