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.

.github/workflows/ci.yml

@@ -0,0 +1,75 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: tests (py${{ matrix.python-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.10", "3.11", "3.12", "3.13"]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install (with dev extras)
+        run: |
+          python -m pip install --upgrade pip
+          pip install -e ".[dev]"
+      - name: Run test suite
+        run: pytest -q
+
+  smoke-install:
+    name: clean-install smoke
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - name: Fresh install (no dev extras) and import
+        run: |
+          python -m pip install --upgrade pip
+          pip install .
+          # Catches undeclared runtime deps (e.g. #994 python-dotenv): a bare
+          # install must import the package and the CLI module.
+          python -c "import tradingagents, cli.main; print('clean-install import OK')"
+
+  lint:
+    name: ruff (changed files, strict)
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - name: Install ruff
+        run: pip install "ruff>=0.15"
+      - name: Lint only the Python files this PR changes
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        run: |
+          # Strict going-forward gate: the full-repo cleanup is deferred (see
+          # worklog 03/01), so we lint only files this PR adds or modifies.
+          files=$(git diff --name-only --diff-filter=ACM "$BASE_SHA"...HEAD -- '*.py' \
+                  | grep -vE '^(results|worklog)/' || true)
+          if [ -z "$files" ]; then
+            echo "No Python changes to lint."
+            exit 0
+          fi
+          echo "Linting changed files:"
+          echo "$files"
+          ruff check $files