ci: add test/lint/smoke workflow, declare python-dotenv, recommend Python 3.12

GitHub Actions: pytest across Python 3.10-3.13, a clean-install import smoke
that catches undeclared runtime deps, and a strict Ruff gate (standard rule set)
scoped to the files each PR changes. Declares python-dotenv (imported by the CLI
but previously undeclared) and adds a [dev] extra. Recommends Python 3.12 for
setup, verified from a clean isolated install.

README.md

@@ -111,7 +111,7 @@ cd TradingAgents
 
 Create a virtual environment in any of your favorite environment managers:
 ```bash
-conda create -n tradingagents python=3.13
+conda create -n tradingagents python=3.12
 conda activate tradingagents
 ```