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).

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',