Add repo conventions and cron documentation

cron/gmail-unread-poll.md

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+# Gmail Unread Poll
+
+## Purpose
+
+Poll unread Gmail state for triage/review workflows.
+
+## Entrypoint
+
+- Script: `scripts/gmail-unread-poll.sh`
+
+## Schedule
+
+- Recurring or ad hoc, depending on how the scheduler is configured
+- Record the exact active schedule here if it becomes a standing job
+
+## Expected behavior
+
+- Checks unread Gmail state
+- Supports lightweight inbox monitoring/triage workflows
+- May feed reminder or review flows depending on runtime usage
+
+## Notes
+
+Do not commit mailbox contents, auth artifacts, or generated local state.