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/buffer.lua

@@ -136,6 +136,10 @@ local function apply_extmarks(bufnr, line_meta)
       if m.due then
         table.insert(virt_parts, { m.due, due_hl })
       end
+      if m.file then
+        local display = m.file:match('([^/]+:%d+)$') or m.file
+        table.insert(virt_parts, { display, 'PendingFile' })
+      end
       if #virt_parts > 0 then
         for p = 1, #virt_parts - 1 do
           virt_parts[p][1] = virt_parts[p][1] .. '  '
@@ -170,6 +174,7 @@ local function setup_highlights()
   vim.api.nvim_set_hl(0, 'PendingDone', { link = 'Comment', default = true })
   vim.api.nvim_set_hl(0, 'PendingPriority', { link = 'DiagnosticWarn', default = true })
   vim.api.nvim_set_hl(0, 'PendingRecur', { link = 'DiagnosticInfo', default = true })
+  vim.api.nvim_set_hl(0, 'PendingFile', { link = 'Directory', default = true })
 end
 
 local function snapshot_folds(bufnr)