Isolation tooling: one workspace tool

The existing harness workspace (with a root npm alias) is fixed and extended rather than replaced: dependencies with npm ci, a registry in the git common directory, a fast-forward land. Owned-change composition (its general fixes kept), seed-experiment.mjs and the dead harness integrate are deleted. Checkpoint capture/verify/restore stays only until the dirty tree is adopted, then is deleted. Agent-made candidate clones are deleted once unused; the Codex evidence directory is kept. Until A5 lands, writers qualify in a candidate checkout (engine/scripts/tooling/candidate-checkout.ts).

Carried out by A5 (2026-09-30): the flow is MERGE-GUIDE.md §1b; the dirty tree was adopted by the step-6 commit, so the checkpoint is deleted. Open: the candidate checkout’s retirement once the first workspace lands (A5-CANDIDATE-RETIRE, for the coordinator).