Add DeepSeek preset for agent loop benchmarks

[SonAIengine]

Summary

• add --llm-preset deepseek to the agent-loop benchmark runner
• resolve DeepSeek defaults to https://api.deepseek.com/v1, deepseek-v4-flash, and DEEPSEEK_API_KEY
• preserve explicit CLI overrides and keep the existing local defaults for the local preset

Why

This avoids accidentally falling back to local/Qwen defaults when the intended quality path is DeepSeek Flash.

Verification

• uv run pytest tests/test_agent_search_benchmarks.py tests/test_agent_efficiency.py tests/test_sufficiency_gate.py
• uv run ruff check examples/ablation/run_agent_loop_benchmarks.py tests/test_agent_search_benchmarks.py tests/test_agent_efficiency.py src/synaptic/agent_loop.py
• git diff --check