Cron scheduler

The runtime includes a per-worker cron scheduler that fires every second to check for due jobs. Cron jobs registered via cron() in scripts are:

• Stored in a per-worker registry keyed by guild ID
• Evaluated using the croner crate (POSIX/Vixie-cron compatible)
• Dispatched as synthetic events (__cron:<name>) through the same dispatch path as Discord events
• Subject to their own timeout (cron_timeout_secs, default 5s)
• Limited per guild (max_cron_jobs, default 32)

Cron jobs are cleared automatically when a guild script is redeployed or unloaded.

...document others later