refactor: break up agent_node.py into focused modules

- Extract input_resolver.py: input path resolution and validation
- Extract execution_engine.py: FFmpeg pipeline execution and retry
- Extract output_handler.py: frame collection and formatting
- Extract batch_processor.py: batch/queue processing
- Extract nollm_modes.py: no-LLM mode presets
- agent_node.py remains as thin orchestrator calling these modules

Author: AEmotionStudio