A removed session can no longer be the delivery owner of its continuable
children, but the `host/session-removed` handler only reconciled the
removed row's own activity. `parentAvailable` was updated exclusively from
`refreshSubagents` success, and removal schedules no catalog refresh — so
after the parent's Activation detaches, an addressed child kept a writable
editor against a dead continuation owner until an unrelated refresh (or
forever, for a closed menu).
Flip `parentAvailable` to false on the owned catalog and push
`handleSubagentParentAvailable(false)` to every addressed child Session at
removal time, matching the refresh path's notification. New Session
instances already read `parentAvailable` from the catalog, so they inherit
the invalidated state.
Adds a regression test: removing the catalog's owning parent flips the
snapshot's `parentAvailable` and notifies the addressed child instance.
Tianyi Cui
8431dbead3