feat(devcontainer): add Codespaces config for zero-config onboarding

Camila Maia pattern — a contributor clicks "Code -> Codespaces" and
gets a working environment without debugging Python / libCST on
their own machine.

Setup:
- Python 3.11 base image (matches CI)
- postCreate: pip install -e '.[python]' so the libCST-based scanner
  is available alongside the core package; plus pytest, ruff,
  pre-commit; plus pre-commit install so commits get formatted.
- VS Code extensions: Python + Ruff. No heavy weights, no Streamlit
  or Jupyter — sql-sop is a CLI / library, the container reflects
  that.

After a successful Codespace boot a contributor can:
- run ``pytest -q`` (78 tests)
- run ``sql-sop check tests/fixtures/errors.sql`` to see the tool
  fire on a bad fixture
- edit a rule, save, and pre-commit will ruff-format on commit

Author: Pawansingh3889

.devcontainer/devcontainer.json

@@ -0,0 +1,20 @@
+{
+  "name": "sql-sop",
+  "image": "mcr.microsoft.com/devcontainers/python:3.11",
+  "postCreateCommand": "pip install --upgrade pip && pip install -e '.[python]' && pip install pytest ruff pre-commit && pre-commit install || true",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "ms-python.python",
+        "charliermarsh.ruff"
+      ],
+      "settings": {
+        "python.defaultInterpreterPath": "/usr/local/bin/python",
+        "[python]": {
+          "editor.formatOnSave": false
+        }
+      }
+    }
+  },
+  "remoteUser": "vscode"
+}