ci: add release workflow — build and upload binary on tag push

Triggers on v* tags. Builds .#irish via nix, smoke tests it, then
uploads irish-linux-x86_64 to the GitHub release via gh CLI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Vaniell0

.github/workflows/release.yml

@@ -0,0 +1,35 @@
+name: Release
+
+on:
+  push:
+    tags: ['v*']
+
+jobs:
+  build-and-release:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/install-nix-action@v27
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+
+      - name: Build irish
+        run: nix build .#irish
+
+      - name: Smoke test
+        run: |
+          ./result/bin/irish -e 'ls | head 3 | print'
+          ./result/bin/irish -e '@base.types | print'
+
+      - name: Upload release binary
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          cp result/bin/irish irish-linux-x86_64
+          gh release upload "${{ github.ref_name }}" irish-linux-x86_64 \
+            --clobber