Review finding (MAJOR-2): with dashboard.turn_isolation the plugin's agent-side
code (tools, hooks) runs in the compute-host child, where _live_transports is
empty and _broadcast_global_event dropped the frame at logger.debug. The child
now registers its _HostTransport as a live transport for the life of run_host,
so a session-less broadcast rides the existing host pipe; the parent bridge
(_relay_compute_host_rpc) recognises an event frame with no session_id and
fans it out via _broadcast_global_event instead of write_json, which would
have dropped it on hermes serve's stdio.
Minor: the late `from tui_gateway.server import …` could raise ImportError in
a plugin-only process despite the "safe from any handler" docstring — caught
and logged as a warning.
Docs: the SDK page now tables exactly which process each call site runs in
and what it reaches (hermes serve / compute-host child / stdio TUI / gateway
run + chat + cron = nobody).
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