Summary

The Desktop/TUI websocket disconnect path parks a detached session on the drop transport and schedules a one-shot grace timer (_schedule_ws_orphan_reap). _ws_session_is_orphaned deliberately spares sessions that are mid-turn — but when the in-flight turn outlives the grace window (long tool calls, context compression, the post-turn background review pass), the timer fired once, saw running=True, returned, and nothing ever re-checked. The session then sat detached with its DB row un-ended until the multi-hour idle reaper (_SESSION_TTL_S = 6h default), so the compression continuation child showed up as a separate "active" Desktop chat alongside the user's real workflow — the exact ended_at = NULL ghost row in the report.

_reap now re-arms itself while the session is detached, un-finalized, and still running. A reconnect or session.resume that re-binds a live transport still cancels the chain exactly as before, and since _finalize_session already ends agent.session_id (the rotated continuation id, #20001), the reap lands on the right DB row with end_reason=ws_orphan_reap.

Fixes #44045

Why this addresses the reported shape

• parent end_reason='compression', child ended_at=NULL: compression rotates the session id mid-turn; the client had already disconnected, so the spared-once reap never came back for the continuation.
• gateway active_sessions: 1: the ghost is not running anything — it's only parked in _sessions and projected as active via its un-ended DB row.

Testing

Two new tests in tests/test_tui_gateway_server.py (pattern-matched to the existing orphan-reap tests, no real timers):

• test_ws_orphan_reap_rearms_while_detached_session_is_mid_turn — grace expires mid-turn → spared and re-armed; turn ends → re-armed timer reaps with ws_orphan_reap; no further re-arm. Fails on main (timer never re-arms), passes with the fix.
• test_ws_orphan_reap_does_not_rearm_after_reattach — a mid-turn session that re-bound a live transport stops the chain (guards the reconnect behavior).

tests/test_tui_gateway_server.py + tests/test_tui_gateway_ws.py: 260 passed, 1 pre-existing environment-dependent failure (test_browser_manage_connect_default_local_reports_launch_hint, fails identically on clean main).