M3b plan: task 11's skeleton writes run as glue over small helpers

The third attempt filled five functions, then ran out of room planning all of
run in one turn. run and run_container are now given; spawn, Io::start,
Io::finish and answer are small todo!()s. Checked fillable: 11 of 11 passed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M3b/12-brokerd-egress.md

@@ -33,9 +33,13 @@ impl Podman {
 
 After step 2 of task 11 (name, input, limit):
 
-- `spec.egress()` is `None` → exactly as before: `tool_args(…, None)` and run the container.
-- `spec.egress()` is `Some(hosts)` → start the proxy (below), and on success run the container
-  with `tool_args(spec, &runner, &name, Some(&dir))`.
+- `spec.egress()` is `None` → exactly as before: `tool_args(…, None)` and `run_container`.
+- `spec.egress()` is `Some(hosts)` → start the proxy (below), and on success call
+  `run_container` with `tool_args(spec, &runner, &name, Some(&dir))`.
+
+`run_container` (from task 11) does steps 3 to 7 for any argument list, so the only change to
+`run` is this choice. Put the proxy's start in its own function (`start_egress`), returning the
+guard or the error.
 
 ### Starting the proxy: every step and exit