feat(file-token): file: inline metadata token with gf navigation (#45)

* 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.

* test(file-token): add parse, diff, views, edit, and navigation tests

Problem: the file: token implementation had no test coverage.

Solution: add spec/file_spec.lua covering parse.body extraction,
malformed token handling, duplicate token stop-parsing, diff
reconciliation (store/update/clear/round-trip), LineMeta population
in both views, :Pending edit -file, and goto_file notify paths for
no-file and unreadable-file cases. All 292 tests pass.

* style: apply stylua formatting

* fix(types): remove empty elseif block, fix file? annotation nullability

lua/pending/views.lua

@@ -12,6 +12,7 @@ local parse = require('pending.parse')
 ---@field show_category? boolean
 ---@field priority? integer
 ---@field recur? string
+---@field file? string
 
 ---@class pending.views
 local M = {}
@@ -159,6 +160,7 @@ function M.category_view(tasks)
         overdue = task.status == 'pending' and task.due ~= nil and parse.is_overdue(task.due)
           or nil,
         recur = task.recur,
+        file = task._extra and task._extra.file or nil,
       })
     end
   end
@@ -210,6 +212,7 @@ function M.priority_view(tasks)
       overdue = task.status == 'pending' and task.due ~= nil and parse.is_overdue(task.due) or nil,
       show_category = true,
       recur = task.recur,
+      file = task._extra and task._extra.file or nil,
     })
   end