[finish] Fix October broken sessions root causes

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 7 coders (9464–9470) done, PRs #6762/64/65/66/68/69/70 merged to develop; 5 root causes covered (1c already shipped via Task 9264). No follow-ups. Owed: prod 5-day oracle after master promotion. Memory: none applied.