feat: Run job immediately on add (ScheduleEntryNow)

[CybotTM]

Summary

Add option to run a job immediately when it's added, rather than waiting for the first scheduled time.

Upstream Reference

• PR: feat: add ScheduleEntryNow function to trigger entry immediately outside of schedule robfig/cron#511 - ScheduleEntryNow

Related Issues

• This is related to feat: Add WithPrev and WithRunImmediately entry options #221 (WithPrev option from feat: support overriding the Prev time for a new job/entry robfig/cron#446) which tracks previous execution time
• This is related to feat: Add schedule.Prev() method for previous execution time #222 (WithRunImmediately from add schedule.Prev() robfig/cron#361)

Note: #221 and #222 may already cover this use case. This issue tracks the specific ScheduleEntryNow approach from PR robfig#511.

Problem

When adding a job that runs infrequently (e.g., daily), users often want it to execute immediately for:

• Initial data population
• Testing/validation
• Ensuring timely first execution

Proposed API

// Option 1: ScheduleEntryNow function
c.ScheduleEntryNow(entry)

// Option 2: AddFunc with immediate flag
c.AddFunc("@daily", myFunc, cron.WithImmediate())

// Option 3: Entry method
entry := c.Entry(id)
entry.RunNow()

Priority

Medium - Common requirement for job initialization.

Implementation Notes

Consider interaction with SkipIfStillRunning and DelayIfStillRunning wrappers.

[CybotTM]

Closing as duplicate of #221.

Issue #221 covers both WithPrev and WithRunImmediately entry options which address the same use case as ScheduleEntryNow (PR robfig#511). The WithRunImmediately API in #221 is cleaner and more consistent with our existing entry options pattern.

See also #222 which tracks the same feature from a different upstream PR.