docs(testing): define Nix and tmachine target state

[elezar]

Summary

Define the desired OpenShell testing model and the migration from host-managed and mise-managed integration tests to Nix-built artifacts and tmachine.

The document separates source validation, merge integration, specialized suites, and candidate-artifact release validation while recording the decisions that remain deferred.

Related Issue

No issue required: this is a documentation-only target-state clarification requested directly by a maintainer.

Changes

• Define Nix environments for source checks and tmachine for integration and Linux installation validation
• Separate platform, environment, configuration, installation, and testsuite axes
• Define conformance, feature-specific, and driver-specific integration suites
• Specify the initial fixed merge matrix and conditional SDK and packaging checks
• Define candidate RPM, DEB, Snap, Helm, and existing Homebrew release-validation paths
• Document the default local Ubuntu/Docker conformance cycle
• Add a phased migration plan and an explicit deferred-decisions list
• Relate the desired model to the current CI workflows and tmachine implementation

Testing

• mise run pre-commit passes (not run: mise is unavailable in this environment)
• Markdown lint passes for TESTING.md and CI.md using markdownlint-cli2 from Nix
• git diff --check passes
• nix flake show --no-write-lock-file passes
• Unit tests added/updated (not applicable: documentation only)
• E2E tests added/updated (not applicable: documentation only)

Checklist

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

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.