feat: replace per-agent BM25 memory with persistent append-only decision log

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tradingagents/default_config.py

@@ -6,6 +6,7 @@ DEFAULT_CONFIG = {
     "project_dir": os.path.abspath(os.path.join(os.path.dirname(__file__), ".")),
     "results_dir": os.getenv("TRADINGAGENTS_RESULTS_DIR", os.path.join(_TRADINGAGENTS_HOME, "logs")),
     "data_cache_dir": os.getenv("TRADINGAGENTS_CACHE_DIR", os.path.join(_TRADINGAGENTS_HOME, "cache")),
+    "memory_log_path": os.path.join(_TRADINGAGENTS_HOME, "memory", "trading_memory.md"),
     # LLM settings
     "llm_provider": "openai",
     "deep_think_llm": "gpt-5.4",