[Feature]: Active time window for interval scheduled tasks #26116

Problem

Recurring scheduled tasks can only be interval (every N ms, around the clock) or fixed_time (one wall-clock time, optional weekdays). There is no way to say "run every hour, but only between 08:00 and 19:00".

My real use case: a recurring task that should run during working hours only, e.g. every 30–60 min from 08:00 to 19:00 on weekdays. Today the options are:

1. One fixed_time task per slot: 12 tasks for hourly 08–19, 24 for every 30 min. Works, but it's a pain to create, edit, pause, or change the prompt for all of them at once.
2. interval plus a guard in the prompt ("if local time is outside 08:00–19:00, reply 'skipped' and stop"): one task, but every off-hours tick still starts an agent turn and spends usage/tokens just to decide not to work. It also fills the thread with no-op turns.

Proposal

Add an optional active window to interval schedules:

{
  "type": "interval",
  "everyMs": 1800000,
  "activeWindow": {
    "start": "08:00",          // local wall-clock, inclusive
    "end": "19:00",            // local wall-clock, exclusive; end < start could mean an overnight window
    "weekdays": [1, 2, 3, 4, 5] // optional, same convention as fixed_time (0 = Sunday)
  }
}

Behavior:

• Ticks that fall outside the window are skipped by the scheduler, so no turn starts and no usage is spent.
• nextRunAt jumps to the next in-window slot (e.g. Friday 19:00 → Monday 08:00).
• Ideally the first run of each window fires at start (08:00) rather than "whenever the interval happens to land", i.e. intervals align to the window start.
• Uses the same local time zone as fixed_time (related: scheduler: scheduled task times shown in UTC instead of local/system time (IST) #18632).

UI: in the schedule editor, add a "Only between [08:00] and [19:00] on [weekdays]" row under the interval picker. MCP: expose the same field on schedule_task / update_scheduled_task.

Alternatives considered

• Allowing multiple timeOfDay values on fixed_time (["08:00","09:00",...]) would also help, but an interval plus window is easier to express and edit.
• Full cron syntax (*/30 8-18 * * 1-5) covers this too, but is less friendly in the UI. It could be an advanced option.

Thanks!