feat(crew): add on_event hook so overrides can't silence the agent

[abhishekmishragithub]

Problem

OutputCrewNode.process_event carries the framework routing — SDKSystemLLMRequestEvent -> _handle_llm_request (the LLM turn), transcript -> context, settings updates. A subclass that overrides process_event without super() silently drops all of it and the agent stops responding (the "0 turns, silent" symptom we hit).

Fix (per review)

Extract the routing into a private _route_framework_event() and add an on_event() hook as the documented, super()-free extension point:

async def process_event(self, event):
    await super().process_event(event)      # interrupt handling
    await self._route_framework_event(event) # framework-owned, always runs
    await self.on_event(event)               # user hook — override THIS, no super() needed
    await self.send_event(event)

Overriding on_event can't silence the agent; overriding process_event is discouraged in the docstring.

Scope + backward compat

Contained to OutputCrewNode. RootNode/SinkNode/BackgroundCrewNode legitimately override process_event without super() (no framework routing, and Background deliberately skips interrupts) — untouched, so interrupt semantics don't change anywhere. Legacy process_event-with-super() overrides keep working.

Tests

tests/custom/test_output_node_on_event_hook.py: (1) overriding on_event without super keeps the LLM routing; (2) legacy process_event+super still routes. Both green.

Pairs with #70 (Ready-before-nodes). Both are the SDK-side lifecycle footguns behind why node-level greeting attempts silenced the agent. No version bump — bundle into 5.4.0.

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