[finish] Converge every runner on one agent contract: ACP-native

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.settle` 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 17 build children merged and deployed, but the epic is not working on dev: ACP runs die within seconds and SDK v2 agents fail at launch. The Codex, Gemini and MicroVM cells were never run. Filed follow-ups 7467, 7468 and 7469.