Adds an allow_session flag to the gateway approval payload so adapters
can render the session tier independently of the permanent tier. Matrix
gains a session reaction (🌀) and a reaction legend; pure-tirith prompts
now offer once/session/deny instead of collapsing to once/deny.
Salvaged from PR #67312, adapted to the allow_permanent semantics that
landed in #68597 (Always offered when any dangerous-pattern warning is
persistable; pure-tirith prompts stay session-max).
