LA-01: Preserve conversation roles #95

Part of #84.

Owner: Anders (@andersalm).

Depends on LA-00 (#94). Assistant sends an ordered list of system, user and assistant messages through a versioned model contract.

Current progress: PR #75, commit 1754ee9c, passed local checks, independent review and all eight required CI checks in run 36521550976. Source acceptance is complete; PR #75 is merged into feat/0.7.1-models and the combined base is 501b411c. The installed SmolLM2 comparison remains part of target acceptance.

Done means

Release acceptance: Each exposed feature works clearly from start to end, including recovery. Keep unfinished or confusing features hidden until their complete journey passes. Hiding a feature does not complete its task; source checks alone do not establish installed acceptance.

• The engine receives the same roles, order and content that Runtime approved.
• Plain-prompt callers remain supported; capability selection reaches the server and Assistant.
• Changed input under an existing request ID is rejected. Existing terminal, active and uncertain runs retain their original authority and cannot dispatch again during recovery.
• Local activation and retirement still recognize a valid offer. Unpaid hosted and remote fixtures preserve their binding checks.
• Focused tests, the required local gate and independent review pass on the published candidate.