Support selecting specific weekdays for Automations (e.g., Mon/Wed/Fri)

### Summary
Add the ability to schedule an Automation to run on specific weekdays (for example: every Monday, Wednesday and Friday) instead of only the current options to run daily or weekly.

### Motivation
Today the Automations schedule only supports "daily" or "weekly" runs with no way to pick particular days of the week. To run something on multiple specific weekdays users must create multiple automations (one per day) which is confusing and noisy.

Being able to select multiple weekdays would make schedules more expressive and reduce duplication.

### Proposed solution
- Extend the automation schedule UI to allow selecting multiple weekdays when the cadence is weekly. For example a set of checkboxes or a multi-select with: Mon, Tue, Wed, Thu, Fri, Sat, Sun.
- Keep the existing "daily" and "weekly" options for backward compatibility; when "weekly" is chosen, provide the new weekday picker. Provide a clear label like "Run on" followed by the weekday selection.
- Backend/API: accept a daysOfWeek array (e.g. ["monday","wednesday","friday"]) or equivalent field in the automation schedule model and execution logic that triggers runs on those days. Keep current weekly semantics when daysOfWeek is empty to preserve existing behavior.
- Consider exposing the same option in any automation templates or export/import formats so users can programmatically create schedules that include multiple weekdays.

### Examples / Use cases
- Run a cleanup task every Monday, Wednesday and Friday.
- Run a report every weekday (Mon–Fri) without creating five separate automations.

### Acceptance criteria
- The schedule UI allows selecting one or more weekdays for weekly schedules.
- Automation execution logic runs jobs on the selected weekdays only.
- Existing automations without a daysOfWeek field continue to behave as they do today.
- Documentation and API/schema docs updated to show the new field and examples.
- Unit/integration tests cover the scheduler behavior for multiple-day schedules.

### Additional notes
- An advanced option (e.g., cron expression) could be considered separately for power users, but this request focuses on selecting common weekday combinations via a simple UI.

[View original Slack conversation](https://vscodeteam.slack.com/archives/C09MVCZKLP4/p1786117774192499?thread_ts=1786117774.192499&cid=C09MVCZKLP4)