feat: time-aware due dates, persistent undo, @return audit (#33)

* fix(plugin): allow command chaining with bar separator

Problem: :Pending|only failed because the command definition lacked the
bar attribute, causing | to be consumed as an argument.

Solution: Add bar = true to nvim_create_user_command so | is treated as
a command separator, matching fugitive's :Git behavior.

* refactor(buffer): remove opinionated window options

Problem: The plugin hardcoded number, relativenumber, wrap, spell,
signcolumn, foldcolumn, and cursorline in set_win_options, overriding
user preferences with no way to opt out.

Solution: Remove all cosmetic window options. Users who want them can
set them in after/ftplugin/pending.lua. Only conceallevel,
concealcursor, and winfixheight remain as functionally required.

* feat: time-aware due dates, persistent undo, @return audit

Problem: Due dates had no time component, the undo stack was lost on
restart and stored in a separate file, and many public functions lacked
required @return annotations.

Solution: Add YYYY-MM-DDThh:mm support across parse, views, recur,
complete, and init with time-aware overdue checks. Merge the undo stack
into the task store JSON so a single file holds all state. Add @return
nil annotations to all 27 void public functions across every module.

* feat(parse): flexible time parsing for @ suffix

Problem: the @HH:MM time suffix required zero-padded 24-hour format,
forcing users to write due:tomorrow@14:00 instead of due:tomorrow@2pm.

Solution: add normalize_time() that accepts bare hours (9, 14),
H:MM (9:30), am/pm (2pm, 9:30am, 12am), and existing HH:MM format,
normalizing all to canonical HH:MM on save.

* feat(complete): add info descriptions to omnifunc items

Problem: completion menu items had no description, making it hard to
distinguish between similar entries like date shorthands and recurrence
patterns.

Solution: return { word, info } tables from date_completions() and
recur_completions(), surfacing human-readable descriptions in the
completion popup.

* ci: format

lua/pending/diff.lua

@@ -65,6 +65,7 @@ function M.parse_buffer(lines)
 end
 
 ---@param lines string[]
+---@return nil
 function M.apply(lines)
   local parsed = M.parse_buffer(lines)
   local now = timestamp()