feat: todo done accepts multiple task numbers and prints completed task

- Change `todo done <N>` to variadic `todo done <N>...` for stable
  indices across sequential calls
- Process multiple numbers highest-index-first so lower indices remain
  valid as tasks are removed
- Validate all numbers upfront before mutating any files
- After marking done, print the full task block (same format as list)
  so the user gets visual confirmation of what was completed
- Extract `mark_task_done` as a testable helper; add unit tests

src/cli/args.rs

@@ -20,10 +20,11 @@ pub enum TodoAction {
         /// Task number to edit
         number: usize,
     },
-    /// Mark a task as done
+    /// Mark one or more tasks as done
     Done {
-        /// Task number to mark as done
-        number: usize,
+        /// Task numbers to mark as done (processed highest-index-first for stable indices)
+        #[arg(required = true, num_args = 1..)]
+        numbers: Vec<usize>,
     },
 }