feat(filter): oil-like editable filter line (#43)

* 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

lua/pending/views.lua

@@ -2,7 +2,7 @@ local config = require('pending.config')
 local parse = require('pending.parse')
 
 ---@class pending.LineMeta
----@field type 'task'|'header'|'blank'
+---@field type 'task'|'header'|'blank'|'filter'
 ---@field id? integer
 ---@field due? string
 ---@field raw_due? string