feat(file-token): add file: inline metadata token with gf navigation

Problem: there was no way to link a task to a specific location in a
source file, or to quickly jump from a task to the relevant code.

Solution: add a file:<path>:<line> inline token that stores a relative
file reference in task._extra.file. Virtual text renders basename:line
in a new PendingFile highlight group. A buffer-local gf mapping
(configurable via keymaps.goto_file) opens the file at the given line.
M.add_here() lets users attach the current cursor position to any task
via vim.ui.select(). M.edit() gains -file support to clear the
reference. <Plug>(pending-goto-file) and <Plug>(pending-add-here) are
exposed for custom mappings.

lua/pending/config.lua

@@ -22,6 +22,7 @@
 ---@field prev_header? string|false
 ---@field next_task? string|false
 ---@field prev_task? string|false
+---@field goto_file? string|false
 
 ---@class pending.Config
 ---@field data_path string