[finish] Multi-Step Signup Flow With Stripe Billing

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 5 coders merged (#6458, #6464, #6462, #6461, #6467); signup is live on dev with ?plan carried and validation working. 2FA moved to first sign-in (BA D3). Filed 8521 (audit regression, Stripe PII purge) and 8522 (UX polish). Memory: none applied.