Preserving aborted refs on dispose exposed a latent invariant break: the
executor's hard-abort path (finalizeSubagentLifecycle) set status `aborted`
and disposed the session without detaching it. With the ref now retained, it
kept a dangling pointer to the disposed session, and ensureLive returns any
non-null ref.session before its revivability check — so hub focus / transcript
chat could route into a dead session.
- finalizeSubagentLifecycle: detach the session before disposing on the
terminal hard-abort path, upholding the AgentRef invariant (session === null
when aborted).
- release(tombstone): detach before dispose too (capture the live session
first), same invariant.
- unregisterUnlessParked: preserve `aborted` refs only when already detached;
an aborted ref still holding a live session is a bug and is unregistered
rather than kept reachable.
- Regression test now asserts ensureLive rejects a tombstoned id as terminal.
Fixes#7250
roboomp
57732e9dcd