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/__init__.py

@@ -1,5 +1,20 @@
 import warnings
 
+# Load .env files at package import so DEFAULT_CONFIG's env-var overlay
+# (and every llm_clients consumer) sees the user's keys regardless of
+# which entry point started the process. find_dotenv(usecwd=True) walks
+# from the CWD, so the installed `tradingagents` console script picks up
+# the project's .env instead of stepping up from site-packages.
+# load_dotenv defaults to override=False, so it never clobbers values
+# the caller has already exported.
+try:
+    from dotenv import find_dotenv, load_dotenv
+
+    load_dotenv(find_dotenv(usecwd=True))
+    load_dotenv(find_dotenv(".env.enterprise", usecwd=True), override=False)
+except ImportError:
+    pass
+
 # langchain-core 1.3.3 calls surface_langchain_deprecation_warnings() in
 # its own __init__, which prepends default-action filters for its
 # subclassed warning categories. To suppress a specific warning we must