[FEATURE] Cron jobs / reminders should persist across sessions via shared file state

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Problem

Cron jobs (reminders) created via CronCreate are session-scoped and live only in memory. This means:

• Deleting a reminder in session B does not remove it from session A
• Creating a reminder in one session is invisible to other running sessions
• There is no single source of truth for active reminders

Proposed Solution

Proposed solution

Use a file-based shared state (~/.claude/crons.json) as the single source of truth.

• CronCreate → writes to the shared file
• CronDelete → removes/deactivates the entry
• Each session periodically syncs from the file (or uses a file watcher)

Lightweight approach — no IPC needed, just a shared JSON file with basic file locking.

Alternative Solutions

_No response_

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

Related

Focused subset of #24798 (inter-session communication). That covers full agent-to-agent messaging; this is scoped to just making cron/reminder state
consistent across sessions.