Plan-level habit (top-level HABITS block). Distinct from the in-day Habit activity: this represents a coaching cue carried across the whole plan (e.g. weekly weigh-in, daily food log) rather than a measurable target dropped into a day's blocks.
HABITS
Habit
@type t() :: %WplAi.AST.PlanHabit{ description: String.t() | nil, frequency: String.t() | nil, meta: map(), name: String.t(), trigger: String.t() | nil }