feat: Add nix-setup composite action

Reusable action for Nix CI with caching:
- DeterminateSystems/nix-installer-action for Nix installation
- actions/cache@v4 for Nix store caching
- Cache key based on flake.lock for per-repo isolation

nix-setup/action.yml

@@ -0,0 +1,25 @@
+name: 'Nix Setup with Cache'
+description: 'Install Nix and set up store caching'
+
+inputs:
+  cache-name:
+    description: 'Optional cache name prefix for disambiguation'
+    required: false
+    default: ''
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Install Nix
+      uses: DeterminateSystems/nix-installer-action@main
+
+    - name: Cache Nix store
+      uses: actions/cache@v4
+      with:
+        path: |
+          /nix/store
+          /nix/var/nix
+          ~/.cache/nix
+        key: nix-${{ inputs.cache-name }}${{ runner.os }}-${{ hashFiles('flake.lock') }}
+        restore-keys: |
+          nix-${{ inputs.cache-name }}${{ runner.os }}-