Problem: the plugin has no concept of recurring tasks, which is
needed for habits and repeating deadlines.
Solution: add recur.lua with parse(), validate(), next_due(),
to_rrule(), and shorthand_list(). Supports named shorthands (daily,
weekdays, weekly, etc.), interval notation (Nd, Nw, Nm, Ny), raw
RRULE passthrough, and ! prefix for completion-based mode. Includes
day-clamping for month/year advancement.
