prune_sessions hand-rolled the same "guarded by a live lease/lock"
comprehension as the new _guarded_ids helper, so the two could drift on
the next guard change. Move _guarded_ids next to _write_guards_reject in
the maintenance mixin and have prune call it.
delete_session walked the delegate tree up to three times in one write
transaction; compute the target ids once for both the guard check and the
expected-ids fence. delete_sessions ran a per-root guard walk for every
selected id; do one batched check over all roots and their children first
and only attribute per root when something is actually guarded.
The CLI export --delete message repeated "session 'X'" because the
exception text already names the session.
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