Fix cron timezone resolution and delete drift

[jamiepine]

Summary

• add cron_timezone config support at defaults and per-agent levels, plus SPACEBOT_CRON_TIMEZONE env fallback with validation and system-time fallback
• switch active-hour checks to use the resolved timezone, and expose timezone context in cron list API and tool output
• fix cron tool delete behavior to unregister in-memory scheduler timers before deleting from storage
• update cron/config docs to explain timezone precedence and behavior

Testing

• cargo check
• cargo test test_cron_timezone_ --lib
• cargo test hour_in_active_window --lib