Make task tool reminder interval configurable

Summary

The automatic system reminder "The task tools haven't been used recently..." fires too frequently during conversations where task tracking isn't relevant. It interrupts the flow and adds noise to the context window.

Request

Allow the reminder interval to be configurable in settings.json — either as an absolute duration or a multiplier on the default interval. For example:

{
  "taskReminderInterval": "5m"
}

or

{
  "taskReminderMultiplier": 2.5
}

Alternatively, allow disabling the reminder entirely for users who prefer to use task tools on their own initiative.

Why

• Not every conversation benefits from task tracking, and the reminder is disruptive when it's not applicable.
• The reminder includes "ignore if not applicable" but still consumes context and attention.
• Users should be able to tune this to their workflow.