Add nix-ci skill (#8)

Adds a new 'nix-ci' skill that guides agents through setting up CI for
GitHub repositories, offering two options:

- GitHub Actions with nix-quick-install-action + vira
- Vira self-hosted with vira.hs configuration

Closes #8

Author: srid

README.md

@@ -8,6 +8,7 @@ AI skill pack — reusable [SKILL.md](https://opencode.ai/docs/skills/) definiti
 |-------|-------------|
 | [`nix-flake`](./skills/nix-flake/SKILL.md) | Writing flakes with flake-parts, formatter, shell scripts, and language templates |
 | [`nix-haskell`](./skills/nix-haskell/SKILL.md) | Haskell projects with haskell-flake: dependencies, settings, devShell, autoWire |
+| [`nix-ci`](./skills/nix-ci/SKILL.md) | CI setup for GitHub repos — GitHub Actions or Vira |
 | [`vhs`](./skills/vhs/SKILL.md) | Deterministic terminal demo screencasts with VHS and wait patterns |
 
 ## Setup (home-manager)

skills/nix-ci/SKILL.md

@@ -0,0 +1,46 @@
+---
+name: nix-ci
+description: Use this when setting up CI for a GitHub repository — offers GitHub Actions or Vira depending on the project
+---
+
+# CI Setup
+
+When the user asks to set up CI, use the **Ask tool** to prompt them to choose:
+
+1. **GitHub public runners** — CI via GitHub Actions
+2. **Vira self-hosted** — self-hosted CI using [Vira](https://vira.nixos.asia)
+
+## Option 1: GitHub Actions
+
+Use the [install-nix](https://github.com/nixbuild/nix-quick-install-action) action (avoid DetSys actions) and invoke Vira to build:
+
+```yaml
+# .github/workflows/ci.yml
+name: CI
+on: [push, pull_request]
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: nixbuild/nix-quick-install-action@v34
+      - run: nix profile install github:juspay/vira
+      - run: vira ci
+```
+
+## Option 2: Vira Self-Hosted
+
+Vira is already running and pointed at the repo. Create a `vira.hs` file in the repo root to configure the build pipeline. Refer to the **[Vira configuration docs](https://vira.nixos.asia/config)** for the DSL format.
+
+### Rules for generating `vira.hs`
+
+- If the repo is on **GitHub**, set `signoff.enable = True`
+- If **multiple `flake.nix` files** exist (e.g., in subdirectories), add all of them to `build.flakes` with appropriate `overrideInputs` settings
+- Leave `cache.url = Nothing` with a comment `-- TODO: configure Attic cache URL`
+- Use the **Ask tool** to ask the user whether `build.systems` should be set to `["x86_64-linux", "aarch64-darwin"]` for multi-platform builds
+
+After creating `vira.hs`, run `vira ci -b` to verify the configuration.
+