Support cron-like scheduling syntax for automations

Feature request: Support cron-like syntax for automation scheduling

Describe the feature you'd like:

Add first-class support for cron-like scheduling expressions for automations so users can specify exactly when an automation should run (e.g., cron expressions or a cron-like shorthand). This enables more control than simple interval or trigger-based schedules.

Why this is useful / Use cases:
- Run automations at specific times (daily/weekly/monthly) or on complex schedules (e.g., weekdays at 18:00, first Monday of the month).
- Let agents schedule follow-up runs (e.g., land telemetry, set a timer for a few days, wake up and re-check telemetry) without manual intervention.
- Support timezone-aware schedules and optional jitter/backoff for distributed runs.

Proposed behavior and details:
- Accept standard cron expressions (with an optional seconds field) or a constrained cron-like syntax in automation definitions.
- Allow timezone specification per schedule (UTC by default).
- Provide a UI to validate and preview next run times for a given expression.
- Support persistence of automation state across runs so scheduled automations can reuse context if desired (optional toggle).

Examples:
- "0 18 * * 1-5" — run at 18:00 on weekdays
- "0 0 1 * *" — run at midnight on the first of every month

Backwards compatibility and safety:
- Keep existing interval and event-based triggers; cron-like schedules should be additive.
- Provide validation and helpful error messages for invalid expressions.

No breaking changes to current automation definitions are required; this is an additive scheduling option for greater flexibility.

[View original Slack conversation](https://vscodeteam.slack.com/archives/C09MVCZKLP4/p1785949028214349?thread_ts=1785949028.214349&cid=C09MVCZKLP4)