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/diff.lua

@@ -27,8 +27,13 @@ end
 function M.parse_buffer(lines)
   local result = {}
   local current_category = nil
+  local start = 1
+  if lines[1] and lines[1]:match('^FILTER:') then
+    start = 2
+  end
 
-  for i, line in ipairs(lines) do
+  for i = start, #lines do
+    local line = lines[i]
     local id, body = line:match('^/(%d+)/(- %[.%] .*)$')
     if not id then
       body = line:match('^(- %[.%] .*)$')
@@ -65,8 +70,9 @@ function M.parse_buffer(lines)
 end
 
 ---@param lines string[]
+---@param hidden_ids? table<integer, true>
 ---@return nil
-function M.apply(lines)
+function M.apply(lines, hidden_ids)
   local parsed = M.parse_buffer(lines)
   local now = timestamp()
   local data = store.data()
@@ -160,7 +166,7 @@ function M.apply(lines)
   end
 
   for id, task in pairs(old_by_id) do
-    if not seen_ids[id] then
+    if not seen_ids[id] and not (hidden_ids and hidden_ids[id]) then
       task.status = 'deleted'
       task['end'] = now
       task.modified = now