Replace the 5s output_tail poll (which often showed nothing) with a real push
stream. The process registry gains an on_output sink called from its reader
threads with each chunk; the tui_gateway wires it to emit agent.terminal.output
{process_id, chunk} (write_json is _stdout_lock-guarded, so emitting from the
reader thread is safe). The desktop routes chunks by process id straight into
the read-only agent xterm via a small writer registry, with a capped backlog so
a tab opened mid-stream (or reopened) replays what it missed.
Drops the fragile poll/tail path: no session-key matching, no truncation, no
lag — full-fidelity ANSI, env-agnostic (local/docker/ssh).
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