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.

pyproject.toml

@@ -19,6 +19,7 @@ dependencies = [
     "langgraph-checkpoint-sqlite>=2.0.0",
     "pandas>=2.3.0",
     "parsel>=1.10.0",
+    "python-dotenv>=1.0.0",
     "pytz>=2025.2",
     "questionary>=2.1.0",
     "redis>=6.2.0",
@@ -32,6 +33,13 @@ dependencies = [
     "yfinance>=1.4.1",
 ]
 
+[project.optional-dependencies]
+dev = [
+    "ruff>=0.15",
+    "pytest>=8.0",
+    "pytest-subtests>=0.13",
+]
+
 [project.scripts]
 tradingagents = "cli.main:app"
 
@@ -52,3 +60,19 @@ markers = [
 filterwarnings = [
     "ignore::DeprecationWarning",
 ]
+
+[tool.ruff]
+line-length = 100
+target-version = "py310"
+extend-exclude = ["results", "worklog"]
+
+[tool.ruff.lint]
+# Standard "good defaults" rule set (pyflakes + pycodestyle + isort + bugbear +
+# pyupgrade + comprehensions/simplify). Line length (E501) and layout are owned
+# by the formatter; whole-repo `ruff format` adoption is deferred until the
+# open-PR backlog clears, to avoid mass merge conflicts.
+select = ["E", "W", "F", "I", "B", "UP", "C4", "SIM"]
+ignore = ["E501"]
+
+[tool.ruff.lint.per-file-ignores]
+"**/__init__.py" = ["F401"]   # intentional re-exports