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/init.lua

@@ -1,4 +1,5 @@
 local buffer = require('pending.buffer')
+local config = require('pending.config')
 local diff = require('pending.diff')
 local parse = require('pending.parse')
 local store = require('pending.store')
@@ -305,6 +306,16 @@ function M._setup_buf_mappings(bufnr)
       end, opts)
     end
   end
+
+  local goto_key = km.goto_file
+  if goto_key == nil then
+    goto_key = 'gf'
+  end
+  if goto_key and goto_key ~= false then
+    vim.keymap.set('n', goto_key --[[@as string]], function()
+      M.goto_file()
+    end, opts)
+  end
 end
 
 ---@param bufnr integer
@@ -629,6 +640,9 @@ local function parse_edit_token(token)
   if token == '-rec' or token == '-' .. rk then
     return 'recur', vim.NIL, nil
   end
+  if token == '-file' then
+    return 'file_clear', true, nil
+  end
 
   local due_val = token:match('^' .. vim.pesc(dk) .. ':(.+)$')
   if due_val then
@@ -673,10 +687,11 @@ local function parse_edit_token(token)
       .. dk
       .. ':<date>, cat:<name>, '
       .. rk
-      .. ':<pattern>, +!, -!, -'
+      .. ':<pattern>, file:<path>:<line>, +!, -!, -'
       .. dk
       .. ', -cat, -'
       .. rk
+      .. ', -file'
 end
 
 ---@param id_str string
@@ -755,6 +770,9 @@ function M.edit(id_str, rest)
     elseif field == 'priority' then
       updates.priority = value
       table.insert(feedback, value == 1 and 'priority added' or 'priority removed')
+    elseif field == 'file_clear' then
+      updates.file_clear = true
+      table.insert(feedback, 'file reference removed')
     end
   end
 
@@ -766,6 +784,18 @@ function M.edit(id_str, rest)
   end
 
   store.update(id, updates)
+
+  if updates.file_clear then
+    local t = store.get(id)
+    if t and t._extra then
+      t._extra.file = nil
+      if next(t._extra) == nil then
+        t._extra = nil
+      end
+      t.modified = os.date('!%Y-%m-%dT%H:%M:%SZ') --[[@as string]]
+    end
+  end
+
   store.save()
 
   local bufnr = buffer.bufnr()
@@ -776,6 +806,90 @@ function M.edit(id_str, rest)
   vim.notify('Task #' .. id .. ' updated: ' .. table.concat(feedback, ', '))
 end
 
+---@return nil
+function M.goto_file()
+  local bufnr = vim.api.nvim_get_current_buf()
+  if vim.bo[bufnr].filetype ~= 'pending' then
+    return
+  end
+  local lnum = vim.api.nvim_win_get_cursor(0)[1]
+  local meta = buffer.meta()
+  local m = meta and meta[lnum]
+  if not m or m.type ~= 'task' then
+    vim.notify('No task on this line', vim.log.levels.WARN)
+    return
+  end
+  local task = store.get(m.id)
+  if not task or not task._extra or not task._extra.file then
+    vim.notify('No file attached to this task', vim.log.levels.WARN)
+    return
+  end
+  local file_spec = task._extra.file
+  local rel_path, line_str = file_spec:match('^(.+):(%d+)$')
+  if not rel_path then
+    vim.notify('Invalid file spec: ' .. file_spec, vim.log.levels.ERROR)
+    return
+  end
+  local data_dir = vim.fn.fnamemodify(config.get().data_path, ':h')
+  local abs_path = data_dir .. '/' .. rel_path
+  if vim.fn.filereadable(abs_path) == 0 then
+    vim.notify('File not found: ' .. abs_path, vim.log.levels.ERROR)
+    return
+  end
+  vim.cmd.edit(abs_path)
+  local lnum_target = tonumber(line_str) or 1
+  vim.api.nvim_win_set_cursor(0, { lnum_target, 0 })
+end
+
+---@return nil
+function M.add_here()
+  local cur_bufnr = vim.api.nvim_get_current_buf()
+  if vim.bo[cur_bufnr].filetype == 'pending' then
+    vim.notify('Already in pending buffer', vim.log.levels.WARN)
+    return
+  end
+  local cur_file = vim.api.nvim_buf_get_name(cur_bufnr)
+  if cur_file == '' or vim.fn.filereadable(cur_file) == 0 then
+    vim.notify('Not editing a readable file', vim.log.levels.ERROR)
+    return
+  end
+  local cur_lnum = vim.api.nvim_win_get_cursor(0)[1]
+  local data_dir = vim.fn.fnamemodify(config.get().data_path, ':h')
+  local abs_file = vim.fn.fnamemodify(cur_file, ':p')
+  local rel_file
+  if abs_file:sub(1, #data_dir + 1) == data_dir .. '/' then
+    rel_file = abs_file:sub(#data_dir + 2)
+  else
+    rel_file = abs_file
+  end
+  local file_spec = rel_file .. ':' .. cur_lnum
+  store.load()
+  local tasks = store.active_tasks()
+  if #tasks == 0 then
+    vim.notify('No active tasks', vim.log.levels.INFO)
+    return
+  end
+  local items = {}
+  for _, task in ipairs(tasks) do
+    table.insert(items, task)
+  end
+  vim.ui.select(items, {
+    prompt = 'Attach file to task:',
+    format_item = function(task)
+      return '[' .. task.id .. '] ' .. task.description
+    end,
+  }, function(task)
+    if not task then
+      return
+    end
+    task._extra = task._extra or {}
+    task._extra.file = file_spec
+    task.modified = os.date('!%Y-%m-%dT%H:%M:%SZ') --[[@as string]]
+    store.save()
+    vim.notify('Attached ' .. file_spec .. ' to task ' .. task.id)
+  end)
+end
+
 ---@param args string
 ---@return nil
 function M.command(args)