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/complete.lua

@@ -121,6 +121,7 @@ function M.omnifunc(findstart, base)
       { vim.pesc(dk) .. ':([%S]*)$', dk },
       { 'cat:([%S]*)$', 'cat' },
       { vim.pesc(rk) .. ':([%S]*)$', rk },
+      { 'file:([%S]*)$', 'file' },
     }
 
     for _, check in ipairs(checks) do
@@ -162,6 +163,7 @@ function M.omnifunc(findstart, base)
         table.insert(matches, { word = c.word, menu = '[' .. source .. ']', info = c.info })
       end
     end
+  elseif source == 'file' then
   end
 
   return matches