[codex] Make dashboard config writes atomic

[rushillshah]

Summary

• export the shared atomic file writer from lib/fs.js
• route dashboard config saves through the shared atomic writer instead of direct writeFileSync
• add a regression guard for the dashboard config write path

Partially addresses #11.

Validation

• npm run check
• npm test
• node ./bin/digital-brain.js ui </dev/null exits with the expected non-interactive terminal message
• npm pack --dry-run

Risk

Low. The change is limited to the dashboard config save path and preserves the existing chmod behavior. The larger source registry / args dedupe from #11 is intentionally left for a separate PR.

[rushillshah]

Maintenance reconciliation note (2026-07-01): CI is passing and the branch is mergeable, but this PR is still draft and review-required. Next action: mark ready/request review before merge.