feat: add VSCode devcontainer matching CI environment

Contributors now get a one-click dev environment that mirrors CI exactly:
Python 3.11 (middle of the 3.9/3.11/3.13 matrix), ruff pinned to the same
>=0.4.0,<0.5 range CI enforces, and pre-commit hooks auto-installed from
the existing .pre-commit-config.yaml.

Pinning ruff in post-create.sh is the load-bearing piece: pyproject only
sets a floor, so without the pin the ruff extension would install 0.15.x
and phantom-fail lint against CI's 0.4.x.

.devcontainer/devcontainer.json

@@ -0,0 +1,25 @@
+{
+  "name": "MemPalace",
+  "image": "mcr.microsoft.com/devcontainers/python:3.11",
+  "features": {
+    "ghcr.io/devcontainers/features/github-cli:1": {}
+  },
+  "postCreateCommand": "bash .devcontainer/post-create.sh",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "ms-python.python",
+        "ms-python.debugpy",
+        "charliermarsh.ruff"
+      ],
+      "settings": {
+        "python.defaultInterpreterPath": "/usr/local/bin/python",
+        "python.testing.pytestEnabled": true,
+        "python.testing.pytestArgs": ["tests/", "-v", "--ignore=tests/benchmarks"],
+        "ruff.importStrategy": "fromEnvironment",
+        "editor.formatOnSave": true,
+        "editor.defaultFormatter": "charliermarsh.ruff"
+      }
+    }
+  }
+}