[finish] Trial customer agent usage tracking and upgrade

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 4 coders merged (#6611/#6614/#6616/#6618), BA/arch/plan settled; meter + Upgrade reachable and clean on dev. Gap: Upgrade prefills 1 below 3 bought → follow-up 8957. Peak/warning not dev-exercised (no fixture node), test-covered. Memory: none applied.