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/diff.lua

@@ -12,6 +12,7 @@ local store = require('pending.store')
 ---@field due? string
 ---@field rec? string
 ---@field rec_mode? string
+---@field file? string
 ---@field lnum integer
 
 ---@class pending.diff
@@ -57,6 +58,7 @@ function M.parse_buffer(lines)
           due = metadata.due,
           rec = metadata.rec,
           rec_mode = metadata.rec_mode,
+          file = metadata.file,
           lnum = i,
         })
       end
@@ -133,6 +135,19 @@ function M.apply(lines, hidden_ids)
           task.recur_mode = entry.rec_mode
           changed = true
         end
+        local old_file = (task._extra and task._extra.file) or nil
+        if entry.file ~= old_file then
+          task._extra = task._extra or {}
+          if entry.file then
+            task._extra.file = entry.file
+          else
+            task._extra.file = nil
+            if next(task._extra) == nil then
+              task._extra = nil
+            end
+          end
+          changed = true
+        end
         if entry.status and task.status ~= entry.status then
           task.status = entry.status
           if entry.status == 'done' then