[codex] Add group channel digest automation template

[SebastianBoehler]

Summary

• add a paused studyos-group-channel-digest Codex automation TOML plus adjacent memory
• scope Discord discovery to the configured StudyOS guild (1501971751247024228) and source channels named group-* only
• keep the automation as a daily cron job and make it human-in-the-loop: propose summaries in each group channel first, then post to #updates only after a non-bot group member explicitly approves
• seed repo codex/automations into $CODEX_HOME/automations from Dockerfile.agent startup without overwriting existing user automation files

Validation

• ruff check .
• pyright
• pytest

Notes

The automation stays paused. It does not create issues, branches, PRs, or merges. Pending proposals and scanned message IDs are automation runtime state, so a group can approve or reject before anything reaches the shared #updates channel.