Add reusable Copilot setup workflow with Nix and UV caching

.github/workflows/copilot-setup-steps.yml

@@ -0,0 +1,36 @@
+name: Copilot Setup Steps
+
+on:
+  workflow_call:
+  workflow_dispatch:
+  pull_request:
+    paths:
+      - '.github/workflows/copilot-setup-steps.yml'
+
+jobs:
+  setup:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+      - uses: nicknovitski/nix-develop@v1
+
+      - name: Cache UV dependencies
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cache/uv
+            .venv
+          key: ${{ runner.os }}-uv-${{ hashFiles('uv.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-uv-
+
+      - name: Install dependencies
+        run: uv sync --extra dev