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

tradingagents/default_config.py

@@ -2,7 +2,45 @@ import os
 
 _TRADINGAGENTS_HOME = os.path.join(os.path.expanduser("~"), ".tradingagents")
 
-DEFAULT_CONFIG = {
+# Single source of truth for env-var → config-key overrides. To expose
+# a new config key for environment-based override, add a row here — no
+# entry-point script changes required. Coercion is driven by the type
+# of the existing default, so users can keep writing plain strings in
+# their .env file.
+_ENV_OVERRIDES = {
+    "TRADINGAGENTS_LLM_PROVIDER":         "llm_provider",
+    "TRADINGAGENTS_DEEP_THINK_LLM":       "deep_think_llm",
+    "TRADINGAGENTS_QUICK_THINK_LLM":      "quick_think_llm",
+    "TRADINGAGENTS_LLM_BACKEND_URL":      "backend_url",
+    "TRADINGAGENTS_OUTPUT_LANGUAGE":      "output_language",
+    "TRADINGAGENTS_MAX_DEBATE_ROUNDS":    "max_debate_rounds",
+    "TRADINGAGENTS_MAX_RISK_ROUNDS":      "max_risk_discuss_rounds",
+    "TRADINGAGENTS_CHECKPOINT_ENABLED":   "checkpoint_enabled",
+}
+
+
+def _coerce(value: str, reference):
+    """Coerce env-var string to the type of the existing default value."""
+    if isinstance(reference, bool):
+        return value.strip().lower() in ("true", "1", "yes", "on")
+    if isinstance(reference, int) and not isinstance(reference, bool):
+        return int(value)
+    if isinstance(reference, float):
+        return float(value)
+    return value
+
+
+def _apply_env_overrides(config: dict) -> dict:
+    """Apply TRADINGAGENTS_* env vars to the config dict in-place."""
+    for env_var, key in _ENV_OVERRIDES.items():
+        raw = os.environ.get(env_var)
+        if raw is None or raw == "":
+            continue
+        config[key] = _coerce(raw, config.get(key))
+    return config
+
+
+DEFAULT_CONFIG = _apply_env_overrides({
     "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")),
@@ -62,4 +100,4 @@ DEFAULT_CONFIG = {
     "tool_vendors": {
         # Example: "get_stock_data": "alpha_vantage",  # Override category default
     },
-}
+})