* feat(filter): oil-like editable filter line with predicate dispatch
Problem: no way to narrow the pending buffer to a subset of tasks
without manual scrolling; filtered-out tasks would be silently deleted
on :w because diff.apply() marks unseen IDs as deleted.
Solution: add a FILTER: line rendered at the top of the buffer when a
filter is active. The line is editable — :w re-parses it and updates
the hidden set. diff.apply() gains a hidden_ids param that prevents
filtered-out tasks from being marked deleted. Predicates: cat:X,
overdue, today, priority (space-separated AND). :Pending filter sets
it programmatically; :Pending filter clear removes it.
* ci: format
