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/main.rs

@@ -12,7 +12,7 @@ fn main() -> miette::Result<()> {
         }) => match action {
             None => streamd::cli::commands::todo::run_list(show_future)?,
             Some(TodoAction::Edit { number }) => streamd::cli::commands::todo::run_edit(number)?,
-            Some(TodoAction::Done { number }) => streamd::cli::commands::todo::run_done(number)?,
+            Some(TodoAction::Done { numbers }) => streamd::cli::commands::todo::run_done(&numbers)?,
         },
         Some(Commands::Edit { number }) => streamd::cli::commands::edit::run(number)?,
         Some(Commands::Timesheet { decimal, debug }) => {