Investigate Slack DM target lookup failure in standup delivery

The Daily Standup cron has been falling back to automatic delivery because Slack DM target lookup fails. Observed in memory/standups/2026-05-22.md (delivery section reads: "Standup posted as final reply (Slack DM target lookup failed — falling back to automatic delivery)"). At least one prior occurrence earlier in the week. Not urgent — the standup still gets delivered via fallback — but worth triaging before it erodes. Next steps: 1. Check the standup cron payload in ~/.openclaw/cron/jobs.json for the intended Slack DM target. 2. Verify the Slack accountId/target (D0ATA8PSWUB for Sergey DM) is still valid via message tool member-info. 3. Look at the standup run log (~/.openclaw/cron/runs/3d6638a5-4bda-406b-9ede-bb72a5f6c0a6.jsonl) for the underlying error. 4. If the target lookup is genuinely broken, file or fix; if it is transient, document and close.