feat(node): add agent_llamaindex — LlamaIndex single-agent node

[dsapandora]

What

Add a RocketRide agent node that runs the LlamaIndex agent framework. Exposes LlamaIndex as a ["agent","tool"] node (questions → answers, invocable as <nodeId>.run_agent) for single-agent execution and hierarchical orchestration.

[github-actions]

🤖 Internal: Discord sync marker

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