A live-session hub kill did not stick: release(tombstone) awaited the
wrapped session dispose first, and createAgentSession's unregisterUnlessParked
removed any non-parked ref, so the subsequent detach/setStatus no-oped and the
ref was gone — leaving the reopen resurrection for idle/running agents.
- release(tombstone) now marks the ref `aborted` BEFORE disposing, so the
dispose guard preserves it; the session is detached afterward.
- unregisterUnlessParked now also spares terminal `aborted` refs (matching
the documented "hard-killed, terminal" retention and finalizeSubagentLifecycle).
- Regression test now uses a session stub that mirrors the real wrapped
dispose (unregister unless parked/aborted), so it fails if the tombstone is
set after dispose.
Fixes#7250
roboomp
d350dd8f84