Reapplied onto current main. The branch had drifted ~3348 commits and a trial
merge produced 48 conflict markers, so this is the same change re-landed rather
than a rebase of the old history.
_interrupt_and_clear_session interrupts the running agent without signalling
plugins, so a plugin holding a per-turn external resource — an outbound RPC
waiting on a tool result the loop will never consume — has no way to learn the
turn is gone. Dispatch agent_loop_stopped immediately after
running_agent.interrupt(), gated on a real running agent: the pending-sentinel
/stop path has no in-flight work, so firing there would be noise.
Per review on #27208, the current helper's behaviour is preserved untouched —
multiplex-aware _adapter_for_source() resolution and cached-agent eviction both
still run; the hook is additive and its dispatch failures are swallowed so a
misbehaving plugin cannot break an interrupt.
Tests fail without the change (hook registration and dispatch) and pass with
it. The three failures in tests/hermes_cli/test_plugins.py::TestPluginDiscovery
are pre-existing on this checkout and reproduce with the change stashed.
f361971eed