feat(tests): let external compute drivers run the OpenShell conformance suite #4848

User Story

As a maintainer, I field questions from teams building external compute drivers
against compute_driver.proto. They want to check, in their own CI, that their
driver behaves like the built-in ones.

Problem Statement

There is no supported way for an external compute driver to run OpenShell's
conformance tests against its own gateway. The only option today is running the
CLI conformance suite from a source checkout (see TESTING.md), which isn't
documented for external use and is being replaced by testkit suites.

Impact / Why This Matters

• External driver authors can't tell whether their driver meets the same bar as
  the built-in drivers.
• Without a shared suite, each driver writes its own checks and expectations
  drift.
• We have no single answer to point people at.

Proposed Design

Goal: an external driver can point OpenShell's conformance tests at its own
gateway, from its own CI, without building OpenShell.

Direction, details to be worked out:

• Conformance means behavior every driver must pass. Tests specific to a driver
  or a setup are kept separate.
• The tests can be run against any existing gateway; they don't provision
  environments.
• They are packaged so an external project can get them for a given OpenShell
  version.

Acceptance Criteria

• An external driver's CI can run OpenShell conformance against its own gateway
  without checking out or building OpenShell.
• Docs explain how to do it.

Alternatives Considered

• Document the existing from-source path as the supported way. Works today, but
  ties external CI to building OpenShell and to a suite being replaced.
• Run external drivers in OpenShell's CI. Doesn't scale.

Checklist

• I've reviewed existing issues and the published docs
• This is a design proposal, not a "please build this" request

Related: #3954 (testing strategy), #3460 (RFC 0016), #4773, #4774.