feat(config): TRADINGAGENTS_* env-var overlay for DEFAULT_CONFIG

Adds a single _ENV_OVERRIDES table in default_config.py with type-aware
coercion (str/int/bool), so users can switch llm_provider, deep/quick
models, backend URL, output language, debate rounds, and the checkpoint
flag purely via .env. Centralizes load_dotenv in the package __init__
so the overlay applies for every entry point (CLI, main.py, programmatic).
Drops the hardcoded model assignments and duplicate dotenv loads in
main.py and cli/main.py. Verified live with OpenAI and Gemini.

#602

main.py

@@ -1,23 +1,12 @@
 from tradingagents.graph.trading_graph import TradingAgentsGraph
 from tradingagents.default_config import DEFAULT_CONFIG
 
-from dotenv import find_dotenv, load_dotenv
-
-load_dotenv(find_dotenv(usecwd=True))
-
-# Create a custom config
+# DEFAULT_CONFIG already applies TRADINGAGENTS_* env-var overrides
+# (llm_provider, deep_think_llm, quick_think_llm, backend_url, etc.),
+# so users can switch models or endpoints purely via .env without
+# editing this script. Override individual keys here only when you
+# want a hard-coded value that should ignore the environment.
 config = DEFAULT_CONFIG.copy()
-config["deep_think_llm"] = "gpt-5.4-mini"  # Use a different model
-config["quick_think_llm"] = "gpt-5.4-mini"  # Use a different model
-config["max_debate_rounds"] = 1  # Increase debate rounds
-
-# Configure data vendors (default uses yfinance, no extra API keys needed)
-config["data_vendors"] = {
-    "core_stock_apis": "yfinance",           # Options: alpha_vantage, yfinance
-    "technical_indicators": "yfinance",      # Options: alpha_vantage, yfinance
-    "fundamental_data": "yfinance",          # Options: alpha_vantage, yfinance
-    "news_data": "yfinance",                 # Options: alpha_vantage, yfinance
-}
 
 # Initialize with custom config
 ta = TradingAgentsGraph(debug=True, config=config)