Fix CI: scope ruff check/format to src/ and tests/ only

README.md Python code blocks use 4-space indent (standard markdown convention)
which conflicts with ruff's 2-space project config. Restrict CI ruff to source
and test directories.

Author: acecchini

.github/workflows/ci.yml

@@ -20,10 +20,10 @@ jobs:
       - uses: actions/checkout@v6
       - uses: astral-sh/ruff-action@v3
         with:
-          args: "check"
+          args: "check src/ tests/"
       - uses: astral-sh/ruff-action@v3
         with:
-          args: "format --check --diff"
+          args: "format --check --diff src/ tests/"
 
   codespell:
     runs-on: ubuntu-latest