[finish] Multi-Org Membership with Isolated Organization Switching

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 9 coders merged; ask met (multi-org incl. Operator, isolated active org, account-menu switcher; no flags, switcher deployed to dev). Filed follow-up 5105: unenrolled person can't switch into an MFA-required org. Dev browser check not possible from this node.