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

doc/pending.txt

@@ -268,6 +268,30 @@ COMMANDS                                                    *pending-commands*
 
     `gcal`    Google Calendar one-way push. See |pending-gcal|.
 
+                                                            *:Pending-filter*
+:Pending filter {predicates}
+    Apply a filter to the task buffer. {predicates} is a space-separated list
+    of one or more predicate tokens. Only tasks matching all predicates (AND
+    semantics) are shown. Hidden tasks are not deleted — they are preserved in
+    the store and reappear when the filter is cleared. >vim
+        :Pending filter cat:Work
+        :Pending filter overdue
+        :Pending filter cat:Work overdue
+        :Pending filter priority
+        :Pending filter clear
+<
+    When a filter is active the buffer's first line shows: >
+        FILTER: cat:Work overdue
+<
+    The user can edit this line inline and `:w` to change the active filter.
+    Deleting the `FILTER:` line entirely and saving clears the filter.
+    `:Pending filter clear` also clears the filter programmatically.
+
+    Tab completion after `:Pending filter ` lists available predicates and
+    category values. Already-used predicates are excluded from completions.
+
+    See |pending-filters| for the full list of supported predicates.
+
                                                                *:Pending-undo*
 :Pending undo
     Undo the last `:w` save, restoring the task store to its previous state.
@@ -421,6 +445,47 @@ Queue view: ~                                             *pending-view-queue*
     text alongside the due date virtual text so tasks remain identifiable
     across categories. The buffer is named `pending://queue`.
 
+==============================================================================
+FILTERS                                                       *pending-filters*
+
+Filters narrow the task buffer to a subset of tasks without deleting any data.
+Hidden tasks are preserved in the store and reappear when the filter is
+cleared. Filter state is session-local — it does not persist across Neovim
+restarts.
+
+Set a filter with |:Pending-filter| or by editing the `FILTER:` line: >vim
+    :Pending filter cat:Work overdue
+<
+
+Multiple predicates are separated by spaces and combined with AND logic — a
+task must match every predicate to be shown.
+
+Available predicates: ~
+
+    `cat:X`       Show only tasks whose category is exactly `X`. Tasks with no
+              category (assigned to `default_category`) are hidden unless
+              `default_category` matches `X`.
+
+    `overdue`     Show only pending tasks with a due date strictly before today.
+
+    `today`       Show only pending tasks with a due date equal to today.
+
+    `priority`    Show only tasks with priority > 0 (the `!` marker).
+
+    `clear`       Special value for |:Pending-filter| — clears the active filter
+              and shows all tasks.
+
+FILTER: line: ~                                          *pending-filter-line*
+
+When a filter is active, the first line of the task buffer is: >
+    FILTER: cat:Work overdue
+<
+
+This line is editable. Write the buffer with `:w` to apply the updated
+predicates. Deleting the `FILTER:` line and saving clears the filter. The
+line is highlighted with |PendingFilter| and does not appear in the stored
+task data.
+
 ==============================================================================
 CONFIGURATION                                                *pending-config*
 
@@ -760,6 +825,11 @@ PendingRecur        Applied to the recurrence indicator virtual text shown
                     alongside due dates for recurring tasks.
                     Default: links to `DiagnosticInfo`.
 
+                                                            *PendingFilter*
+PendingFilter       Applied to the `FILTER:` header line shown at the top of
+                    the buffer when a filter is active.
+                    Default: links to `DiagnosticWarn`.
+
 To override a group in your colorscheme or config: >lua
     vim.api.nvim_set_hl(0, 'PendingDue', { fg = '#aaaaaa', italic = true })
 <