chore: add VS Code workspace config and improve CI infrastructure

Add .vscode/ with Biome formatter, Tailwind, Vitest, Playwright, and
Python debug configs. Extract shared pnpm/Node setup into a composite
GitHub Action and add Dependabot and dependency-review workflows.

.github/dependabot.yml

@@ -0,0 +1,52 @@
+version: 2
+
+updates:
+  # npm/pnpm dependencies (root lockfile covers entire monorepo)
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 10
+    groups:
+      production-deps:
+        dependency-type: "production"
+        update-types:
+          - "minor"
+          - "patch"
+      dev-deps:
+        dependency-type: "development"
+        update-types:
+          - "minor"
+          - "patch"
+    ignore:
+      - dependency-name: "*"
+        update-types: ["version-update:semver-major"]
+
+  # Python dependencies (AI sidecar)
+  - package-ecosystem: "pip"
+    directory: "/packages/ai/python"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+
+  # Docker base images
+  - package-ecosystem: "docker"
+    directory: "/docker"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 3
+
+  # GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    groups:
+      actions:
+        patterns:
+          - "*"