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.

plugin/pending.lua

@@ -12,11 +12,13 @@ local function edit_field_candidates()
     dk .. ':',
     'cat:',
     rk .. ':',
+    'file:',
     '+!',
     '-!',
     '-' .. dk,
     '-cat',
     '-' .. rk,
+    '-file',
   }
 end
 
@@ -270,3 +272,11 @@ end)
 vim.keymap.set({ 'n', 'x', 'o' }, '<Plug>(pending-prev-task)', function()
   require('pending.textobj').prev_task(vim.v.count1)
 end)
+
+vim.keymap.set('n', '<Plug>(pending-goto-file)', function()
+  require('pending').goto_file()
+end)
+
+vim.keymap.set('n', '<Plug>(pending-add-here)', function()
+  require('pending').add_here()
+end)