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

README.md

@@ -1,3 +1,41 @@
 # gitea-actions
 
-Reusable Gitea Actions for CI workflows
+Reusable Gitea Actions for CI workflows.
+
+## Actions
+
+### nix-setup
+
+Installs Nix and sets up Nix store caching for Gitea Actions workflows.
+
+**Features:**
+- Uses DeterminateSystems/nix-installer-action for reliable Nix installation
+- Caches `/nix/store`, `/nix/var/nix`, and `~/.cache/nix`
+- Cache key based on `flake.lock` hash for automatic invalidation
+- Each repo gets isolated cache (different flake.lock = different cache)
+
+**Usage:**
+
+```yaml
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: johno/gitea-actions/nix-setup@main
+      - run: nix flake check
+```
+
+**With custom cache prefix:**
+
+```yaml
+- uses: johno/gitea-actions/nix-setup@main
+  with:
+    cache-name: 'my-project-'
+```
+
+**Inputs:**
+
+| Input | Description | Required | Default |
+|-------|-------------|----------|---------|
+| `cache-name` | Optional cache name prefix for disambiguation | No | `''` |