[finish] Support GCP and Azure like AWS

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 10 build children done; 8 PRs merged and deployed to dev. GCP Linux Your Cloud works live. Gaps filed: #8881 AWS launch regression, #8882 AWS wording on GCP, #8883 Filestore refused, #8884 GCP Windows, #8885 Azure untested live. Memory: none applied.