fix(buffer): replace indentexpr with dedicated o/O mappings

Problem: setup_indentexpr always returned 0 because no task line
starts with whitespace (the /ID/ prefix begins with /), so the
return 2 branch was dead code. Pressing o or O opened a blank line
at column 0 with no ID prefix, which the diff parser cannot
recognise as a task.

Solution: remove setup_indentexpr and M.get_indent() entirely; add
M.open_line(above) which inserts a two-space stub line and enters
insert mode at the end so the user types directly into the new task
body. The diff layer already handles lines matching ^  .+ as new
tasks. Add o and O buffer-local mappings in init.lua.

lua/pending/init.lua

@@ -61,6 +61,12 @@ function M._setup_buf_mappings(bufnr)
   vim.keymap.set('n', 'U', function()
     M.undo_write()
   end, opts)
+  vim.keymap.set('n', 'o', function()
+    buffer.open_line(false)
+  end, opts)
+  vim.keymap.set('n', 'O', function()
+    buffer.open_line(true)
+  end, opts)
 end
 
 ---@param bufnr integer