docs: respect custom state dir in cron example

[Gujiassh]

Summary

• replace the hardcoded ~/.openclaw/workspace/... cron --message example with the documented ${OPENCLAW_STATE_DIR:-$HOME/.openclaw} path form
• keep the English and Chinese core guides aligned with the $OC / custom-state-dir convention documented earlier in the same files
• scope the PR to the two guide docs only

Validation

• reviewed both updated cron registration examples side by side to confirm they now use ${OPENCLAW_STATE_DIR:-$HOME/.openclaw}
• confirmed the old hardcoded bash ~/.openclaw/workspace/scripts/nightly-security-audit.sh example is gone from the two core guide docs

Notes

• clean replacement for the previously closed #9 lane on current origin/main