Consolidate atomic JSON replacement

[lastobelus]

Closes #144 after this stacked child lands on main.

Stack

1. PR Add private remote attachment retrieval #191 / Let remote agents use canonical Markover #187 → main
2. This PR / Consolidate atomic JSON persistence primitives #144 → t3code/issue-187-slice-d initially, then main after Add private remote attachment retrieval #191 merges

What changed

• added one internal atomic JSON replacement primitive with same-directory exclusive temporary creation, complete-byte flush, private mode enforcement, atomic rename, and cleanup;
• migrated review, workspace, settings, and service-endpoint persistence without moving their queues, recovery, ownership, or shutdown behavior;
• reconciled stale enrichment scope after Remove unused private-enrichment runtime #156 removed that runtime and preserved PR Simplify settings writer topology #161's single-writer settings topology;
• added direct primitive coverage and POSIX-private-mode regressions for the migrated stores.

Validation

• rebased onto PR Add private remote attachment retrieval #191 head 3783351
• npm run lint — passed
• npm run typecheck — passed
• focused build plus 83 primitive/review/workspace/settings/service-endpoint tests — passed
• GitHub CI Verify (Node 24) — passed on head b30bd26
• automated current-head review — completed with thumbs-up and zero review threads
• exact decision-gardener timeout test under Node 24 — passed in isolation
• full npm run ci:local — 861/862 tests passed; the unrelated decision-gardener timeout test is load-sensitive and measured about 522 ms against a 590 ms lower-bound assertion when the full suite ran concurrently
• UI QA intentionally not performed: this is an internal persistence refactor with no UI change