[Feature]: Skip scheduled task occurrences when the bound thread is already running #16393

Problem or use case

A recurring scheduled task can target a thread that the user is actively working in. I want the scheduled occurrence to be skipped when that thread already has an active agent turn, without manually pausing and re-enabling the schedule.

For example, a maintenance task runs every hour in a thread. If a coding task is still running at the hourly trigger, skip that occurrence and try again at the next regular scheduled time.

Proposed solution

Add an optional Skip if thread is running setting for thread-bound scheduled tasks, available in the schedule editor and the scheduler MCP create/update tools.

When enabled:

• Check whether the bound thread has an active turn immediately before dispatch, with a guard that prevents a race with another turn starting.
• If it is busy, skip that occurrence without interrupting the active turn or queueing the scheduled prompt for later.
• Keep the task enabled and advance to its next normal scheduled time, without accumulating missed runs.
• Show a reason such as Skipped: thread was running in the task's run history/status.

Smallest useful scope

An opt-in setting for recurring tasks bound to an existing thread. Tasks that create a fresh thread per run do not need this behavior. Preserve existing behavior when the setting is disabled.

Why this matters

Recurring maintenance and check-in tasks should be able to run only when their thread is idle. A scheduler-level check provides that guarantee before a prompt is dispatched; an instruction inside the scheduled prompt cannot prevent dispatch.

Related requests

Related to the general scheduling request in #7966; this request specifically concerns skipping an occurrence when its target thread is already running.