feat(filter): wire F key and <Plug>(pending-filter) mapping (#53)

* refactor(config): remove legacy gcal top-level config key

Problem: the gcal migration shim silently accepted vim.g.pending = { gcal
= {...} } and copied it to sync.gcal, adding complexity and a deprecated
API surface.

Solution: remove the migration block in config.get(), drop the cfg.gcal
fallback in gcal_config(), delete the two migration tests, and clean up
the vimdoc references. Callers must now use sync.gcal directly.

* ci: fix

* fix(spec): remove duplicate buffer require in complete_spec

* docs(pending): reorganize vimdoc and fix incorrect defaults

Problem: sections were out of logical order — inline metadata appeared
before commands, GCal before its own backend framework, store resolution
duplicated and buried after health check. Two defaults were wrong:
default_category documented as 'Inbox' (should be 'Todo') and the gcal
calendar example used 'Tasks' (should be 'Pendings').

Solution: reorder all 21 sections into onboarding-first flow, add a
CONTENTS table with hyperlinks, fix both incorrect defaults in every
location they appeared, and remove the duplicate STORE RESOLUTION
section.

* feat(filter): wire F key and <Plug>(pending-filter) mapping

Problem: the filter predicate logic, diff guard, _on_write handling,
:Pending filter command, and filter_spec were already implemented, but
there was no buffer-local key to invoke filtering interactively.

Solution: add filter = 'F' to keymaps config and defaults, wire the
filter action in _setup_buf_mappings via vim.ui.input, add
<Plug>(pending-filter), and update the vimdoc (mappings table, Plug
section, config example, and FILTERS section).

doc/pending.txt

@@ -244,6 +244,7 @@ Default buffer-local keys: ~
     `<CR>`        Toggle complete / uncomplete (`toggle`)
     `!`           Toggle the priority flag (`priority`)
     `D`           Prompt for a due date (`date`)
+    `F`           Prompt for filter predicates (`filter`)
     `<Tab>`       Switch between category / queue view (`view`)
     `U`           Undo the last `:w` save (`undo`)
     `o`           Insert a new task line below (`open_line`)
@@ -309,6 +310,10 @@ All motions support count: `3]]` jumps three headers forward. `]]` and
 <Plug>(pending-undo)
     Undo the last `:w` save.
 
+                                                      *<Plug>(pending-filter)*
+<Plug>(pending-filter)
+    Prompt for filter predicates via |vim.ui.input|.
+
                                                    *<Plug>(pending-open-line)*
 <Plug>(pending-open-line)
     Insert a correctly-formatted blank task line below the cursor.
@@ -385,7 +390,8 @@ 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
+Set a filter with |:Pending-filter|, the `F` buffer key, or by editing the
+`FILTER:` line: >vim
     :Pending filter cat:Work overdue
 <
 
@@ -567,6 +573,7 @@ loads: >lua
         priority = '!',
         date = 'D',
         undo = 'U',
+        filter = 'F',
         open_line = 'o',
         open_line_above = 'O',
         a_task = 'at',

lua/pending/config.lua

@@ -21,6 +21,7 @@
 ---@field priority? string|false
 ---@field date? string|false
 ---@field undo? string|false
+---@field filter? string|false
 ---@field open_line? string|false
 ---@field open_line_above? string|false
 ---@field a_task? string|false
@@ -67,6 +68,7 @@ local defaults = {
     priority = '!',
     date = 'D',
     undo = 'U',
+    filter = 'F',
     open_line = 'o',
     open_line_above = 'O',
     a_task = 'at',

lua/pending/init.lua

@@ -243,6 +243,13 @@ function M._setup_buf_mappings(bufnr)
     undo = function()
       M.undo_write()
     end,
+    filter = function()
+      vim.ui.input({ prompt = 'Filter: ' }, function(input)
+        if input then
+          M.filter(input)
+        end
+      end)
+    end,
     open_line = function()
       buffer.open_line(false)
     end,

plugin/pending.lua

@@ -258,6 +258,14 @@ vim.keymap.set('n', '<Plug>(pending-undo)', function()
   require('pending').undo_write()
 end)
 
+vim.keymap.set('n', '<Plug>(pending-filter)', function()
+  vim.ui.input({ prompt = 'Filter: ' }, function(input)
+    if input then
+      require('pending').filter(input)
+    end
+  end)
+end)
+
 vim.keymap.set('n', '<Plug>(pending-open-line)', function()
   require('pending.buffer').open_line(false)
 end)