Fix agentic-rag retrieval limit wiring

[koriyoshi2041]

Summary

• allow RAGMode to accept a retrieval limit
• pass that limit into memory retrieval
• add a no-network unittest covering AgenticRAGMode construction and recall flow

Why

AgenticRAGMode currently constructs RAGMode with k=k, but RAGMode.__init__ does not accept that argument. That makes agentic-rag fail during construction before a benchmark run can start.

Validation

• uv run python -m unittest tests.test_agentic_rag
• uv run python -m compileall src/memory_bench/modes tests/test_agentic_rag.py
• git diff --check

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