Deletes a single time activity (a punch shift or a manual break) from a specified time clock. The activity is identified by its unique ID - the same id returned by GET .../time-activities (and used in PUT). The server resolves which user the activity belongs to; the caller supplies only the time clock and the activity ID. The activity cannot be deleted if it falls on a locked or approved timesheet day; the request fails with HAS_LOCKED_DAYS. Time-off / absence entries are not addressable here and resolve to TIME_ACTIVITY_NOT_FOUND.