test(conformance): run driver suites with cargo

[elezar]

Summary

Run driver conformance through the portable Cargo test suites instead of building and distributing the standalone conformance CLI. Remove the obsolete CLI crate now that all supported paths use the library-backed test suites. This PR is stacked on #3864.

Related Issue

Follow-up cleanup for #2792.

Stack

• test(e2e): remove schema parity campaign #3864 removes the completed parity campaign.
• This PR is based on test(e2e): remove schema parity campaign #3864 and contains the conformance execution changes and obsolete CLI cleanup.

Changes

• Run driver conformance from tests/suites/conformance with Cargo.
• Remove standalone conformance-CLI build and setup steps from branch, driver E2E, binary-build, and release workflows.
• Delete crates/openshell-conformance-cli and its workspace lockfile entry.
• Update test, build, policy-advisor, and test-guest documentation for Cargo tests and installed nextest archives.
• Rename the Nix archive binding to reflect that it packages conformance tests rather than a CLI.

Testing

• mise run pre-commit passes
• cargo test --locked --package openshell-conformance passes (18 tests)
• cargo test --locked --manifest-path tests/suites/conformance/Cargo.toml --package openshell-test-conformance-cli --no-run passes
• nix-instantiate --parse tests/artifacts.nix passes
• Agent infrastructure consistency and public skill discovery checks pass
• Live driver E2E jobs were not run locally; the affected GitHub workflows provide that coverage

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-3866.docs.buildwithfern.com/openshell

[krishicks]

Analysis by agent found some cleanup that's worth addressing:

Once the CLI crate is removed, these openshell-conformance items have no remaining callers:

• SCENARIOS, scenarios() and scenario(name)
• FILE_TRANSFER_SCENARIO and run_file_transfer
• the Scenario::description field
• OpenShellRunner::new
• OpenShellRunner::scenario(), which only its own unit test calls

Without the registry's consumer, a scenario added to SCENARIOS does not run unless a matching test file is also added.