`subscribe_outbound()`, `dispatch_outbound()`, and `stop()` have zero
callers — `ChannelManager._dispatch_outbound()` handles all outbound
routing via `consume_outbound()` directly. Remove the dead methods and
their unused imports (`Callable`, `Awaitable`, `logger`).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>