Issue #195: in coordinator mode, the React TUI and Textual app never
injected `<task-notification>` envelopes after a worker finished.
`run_print_mode` had `_drain_coordinator_async_agents` wired in, but
`ReactBackendHost._process_line` and `TextualApp._process_line` only
called `handle_line` and returned, leaving the contract documented in
the coordinator system prompt (results arrive between turns) unmet.
Extract the drain helpers into `openharness.ui.coordinator_drain` and
invoke `drain_coordinator_async_agents` from both interactive hosts
after `handle_line` when coordinator mode is active. The shared module
also avoids a circular import between `ui.app` and `ui.backend_host`.
José Maia
3e09dde114