Add repo conventions and cron documentation

cron/README.md

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+# Cron / Automation
+
+This directory documents recurring jobs and their schedules.
+
+Goal: make it obvious what runs, when it runs, why it exists, and what script/skill it depends on.
+
+## Layout
+
+- `*.md` — one file per recurring job
+- each file should record:
+  - job name
+  - purpose
+  - schedule
+  - entrypoint command/script
+  - dependencies
+  - expected output/behavior
+  - notes on state or alerting
+
+## Current documented jobs
+
+- `daily-overview.md`
+- `github-notifications.md`
+- `capmetro-monitor.md`
+- `gmail-unread-poll.md`
+
+## Notes
+
+This folder is documentation-first.
+
+It does not need to mirror the exact runtime scheduler format as long as it clearly explains the active pattern and entrypoints.