Add repo conventions and cron documentation

cron/github-notifications.md

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+# GitHub Notifications Check
+
+## Purpose
+
+Review GitHub notifications for PR activity and major releases, with filtering to reduce noise.
+
+## Entrypoint
+
+- Skill: `skills/github-notifications/`
+- Main wrapper: `skills/github-notifications/scripts/cron-wrapper.sh`
+- Check script: `skills/github-notifications/scripts/check.sh`
+- Optional dismissal helper: `skills/github-notifications/scripts/auto-dismiss.sh`
+
+## Schedule
+
+- Recurring
+- Exact active schedule should be recorded here when finalized in the runtime scheduler
+
+## Expected behavior
+
+- Checks notification state
+- Surfaces relevant PR activity
+- Highlights major releases and selected dev builds
+- Uses local state to avoid duplicate alerts
+
+## State
+
+- Local generated state file is ignored from git
+- Current ignored file pattern: `skills/**/state.json`
+
+## Notes
+
+If runtime behavior changes, update this file alongside the skill or wrapper changes.