[finish] Review and unify model provider interfaces

Terminal analytical phase: verify every build subtask has completed and resolved, optionally note UI acceptance evidence, then settle. Dispatched as a read-only runner role (feat/finish-as-runner, ADR-0040) whose settle is server-computed (`task.complete` declaring a `finish` terminal) — the server derives the full finish report and flips this child `-> done`; the runner attests nothing. Readied by the fan-out sweep like its siblings; its dependency edges to every coder keep `dispatchable_tasks` from dispatching it until those deps are done.

All 8 children done; PRs #6561/6562/6563/6566/6568/6573 merged and deployed on dev (Jaah Deploy ad0f8f1ee green). Gap: Antigravity API-key ACP lane never run live on dev → follow-up #8862. F1–F7 and lane drafts sit outside the epic as top-level drafts. Memory: used mem-23382.