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/init.lua

@@ -94,6 +94,47 @@ function M.has_due()
   return c.overdue > 0 or c.today > 0
 end
 
+---@param tasks pending.Task[]
+---@param predicates string[]
+---@return table<integer, true>
+local function compute_hidden_ids(tasks, predicates)
+  if #predicates == 0 then
+    return {}
+  end
+  local hidden = {}
+  for _, task in ipairs(tasks) do
+    local visible = true
+    for _, pred in ipairs(predicates) do
+      local cat_val = pred:match('^cat:(.+)$')
+      if cat_val then
+        if task.category ~= cat_val then
+          visible = false
+          break
+        end
+      elseif pred == 'overdue' then
+        if not (task.status == 'pending' and task.due and parse.is_overdue(task.due)) then
+          visible = false
+          break
+        end
+      elseif pred == 'today' then
+        if not (task.status == 'pending' and task.due and parse.is_today(task.due)) then
+          visible = false
+          break
+        end
+      elseif pred == 'priority' then
+        if not (task.priority and task.priority > 0) then
+          visible = false
+          break
+        end
+      end
+    end
+    if not visible then
+      hidden[task.id] = true
+    end
+  end
+  return hidden
+end
+
 ---@return integer bufnr
 function M.open()
   local bufnr = buffer.open()
@@ -102,6 +143,30 @@ function M.open()
   return bufnr
 end
 
+---@param pred_str string
+---@return nil
+function M.filter(pred_str)
+  if pred_str == 'clear' or pred_str == '' then
+    buffer.set_filter({}, {})
+    local bufnr = buffer.bufnr()
+    if bufnr then
+      buffer.render(bufnr)
+    end
+    return
+  end
+  local predicates = {}
+  for word in pred_str:gmatch('%S+') do
+    table.insert(predicates, word)
+  end
+  local tasks = store.active_tasks()
+  local hidden = compute_hidden_ids(tasks, predicates)
+  buffer.set_filter(predicates, hidden)
+  local bufnr = buffer.bufnr()
+  if bufnr then
+    buffer.render(bufnr)
+  end
+end
+
 ---@param bufnr integer
 ---@return nil
 function M._setup_autocmds(bufnr)
@@ -246,13 +311,27 @@ end
 ---@return nil
 function M._on_write(bufnr)
   local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+  local predicates = buffer.filter_predicates()
+  if lines[1] and lines[1]:match('^FILTER:') then
+    local pred_str = lines[1]:match('^FILTER:%s*(.*)$') or ''
+    predicates = {}
+    for word in pred_str:gmatch('%S+') do
+      table.insert(predicates, word)
+    end
+    lines = vim.list_slice(lines, 2)
+  elseif #buffer.filter_predicates() > 0 then
+    predicates = {}
+  end
+  local tasks = store.active_tasks()
+  local hidden = compute_hidden_ids(tasks, predicates)
+  buffer.set_filter(predicates, hidden)
   local snapshot = store.snapshot()
   local stack = store.undo_stack()
   table.insert(stack, snapshot)
   if #stack > UNDO_MAX then
     table.remove(stack, 1)
   end
-  diff.apply(lines)
+  diff.apply(lines, hidden)
   M._recompute_counts()
   buffer.render(bufnr)
 end
@@ -718,6 +797,8 @@ function M.command(args)
     M.archive(d)
   elseif cmd == 'due' then
     M.due()
+  elseif cmd == 'filter' then
+    M.filter(rest)
   elseif cmd == 'undo' then
     M.undo_write()
   else