test(tmachine): add interactive shell testsuite

[elezar]

Summary

Add a named shell testsuite that prepares a disposable tmachine guest and opens an interactive SSH session for manual debugging. Add a none installer for entering a prepared environment without installing OpenShell.

Related Issue

No issue required: this is a localized developer test-harness enhancement extracted from ongoing Podman test work.

Changes

• Add an opt-in interactive testsuite setting with a backward-compatible default
• Add a shell testsuite and Ansible preparation playbook
• Add a none installer that reuses the prepared setup disk without installing OpenShell
• Connect the host terminal to the guest through tmachine SSH forwarding on port 2222
• Document the interactive shell workflow and commands in the build architecture overview

Interactive shell

Start a prepared Ubuntu Docker guest without installing OpenShell:

nix run .#tmachine -- test ubuntu-docker-rootful none shell

Install the locally staged Debian package before opening the shell:

nix run .#tmachine -- test ubuntu-docker-rootful deb shell

Exit the SSH session to shut down and discard the disposable guest.

Testing

• mise run pre-commit passes
• cargo test --manifest-path tests/tmachine/Cargo.toml
• nix build .#tmachine-config --no-link
• nix run .#tmachine -- test ubuntu-docker-rootful deb shell entered the OpenShell-installed guest, ran id, and exited cleanly
• nix run .#tmachine -- test ubuntu-docker-rootful none shell entered the prepared guest, confirmed OPENSHELL_ABSENT, ran id, and exited cleanly

Checklist

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