Add tugboat preview environment for MR demos

Problem / motivation

Reviewing audit_trail today requires a local install (composer require + drush en + chain configuration + sample writes) before the admin surface is even visible. That is a non-trivial friction tax for issue triage, MR review, and operators evaluating the module.

Proposed resolution

Add a tugboat preview environment so MRs and the default branch automatically build a clickable demo. drupal.org's GitLab integration with tugboat handles the hosting; the module just needs a .tugboat/ directory with the build recipe.

Useful demo content to pre-seed:

• An active audit_trail_chain entity in mode: auto claiming the cron + php channels.
• A provisioned audit_trail_secret backed by a Key entity (config provider, random bytes).
• 50-100 chained rows spanning create / update / delete actions, so the entries list, side-by-side diff, and verifier verdicts have something to render.
• One archived segment so the segments list and archive HMAC display are not empty.
• An admin user with known credentials, surfaced on the preview's landing page.

Remaining tasks

• Write .tugboat/config.yml + provisioning script.
• Iterate on actual builds until composer + drush si + module enablement + seed data + admin login all work end-to-end.
• Document the preview URL on the project page once stable.

Scope / effort

Roughly half a day. Lands as a follow-up after 1.0.0-alpha1; not blocking 1.0.0 stable.