[codex] Cache dashboard snapshot

[rushillshah]

Summary

• cache the dashboard vault snapshot in UI state instead of re-reading config during render/menu construction
• refresh the cached snapshot after child commands and source config writes
• add a regression guard for render-path config reads

Closes #10

Validation

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

Risk

Low. The change is limited to terminal dashboard state handling; actions that mutate vault config explicitly refresh the snapshot before re-rendering.

[rushillshah]

Maintenance reconciliation note (2026-06-28): 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.