docs(rules): add nix-workflow rule

Add comprehensive documentation for Nix usage in the project:
- Development environment setup with flake.nix
- CI workflow using nix profile install via setup-nix action
- Tool configuration and adding new packages
- Build flags for local development

This helps maintain consistency when modifying Nix configuration.

Author: ryoppippi

.claude/rules/nix-workflow.md

@@ -0,0 +1,88 @@
+# Nix Workflow
+
+This rule provides guidance on Nix usage in the StackOne SDK.
+
+## Development Environment
+
+The project uses `flake.nix` to define the development environment with `nix develop`.
+
+### Adding Development Tools
+
+To add a new tool to the development environment, add it to `buildInputs` in `flake.nix`:
+
+```nix
+buildInputs = with pkgs; [
+  # runtime
+  nodejs_24
+  pnpm_10
+
+  # formatting and linting tools
+  oxlint  # includes tsgolint
+  oxfmt
+
+  # your new tool here
+  new-tool
+];
+```
+
+## CI Workflow
+
+CI uses `nix profile install` via the `.github/actions/setup-nix/action.yaml` composite action.
+
+### Adding Tools to CI Jobs
+
+Specify tools in the `tools` input of the setup-nix action:
+
+```yaml
+- name: Setup Nix
+  uses: ./.github/actions/setup-nix
+  with:
+    tools: nodejs_24 pnpm_10 oxlint oxfmt
+```
+
+The action installs packages using:
+
+```bash
+nix profile install --inputs-from . nixpkgs#tool1 nixpkgs#tool2
+```
+
+### CI Tool Configuration
+
+- **Default tools**: `nodejs_24 pnpm_10` (defined in action.yaml)
+- **Skip pnpm install**: Set `skip-pnpm-install: 'true'` for jobs that don't need node dependencies
+
+### Example: Adding a New Tool to Lint Job
+
+```yaml
+lint:
+  runs-on: ubuntu-latest
+  steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+    - name: Setup Nix
+      uses: ./.github/actions/setup-nix
+      with:
+        tools: nodejs_24 pnpm_10 oxlint oxfmt new-tool
+    - name: Run Lint
+      run: pnpm run lint
+```
+
+## Build Flags
+
+Always use these flags when running Nix build commands locally:
+
+```bash
+--print-build-logs --show-trace
+```
+
+Example:
+
+```bash
+nix build --print-build-logs --show-trace
+nix flake check --print-build-logs --show-trace
+```
+
+## Notes
+
+- Some packages bundle multiple tools (e.g., `oxlint` includes `tsgolint`)
+- Check nixpkgs for package contents before adding redundant dependencies

CLAUDE.md

@@ -15,6 +15,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 | **pnpm-usage**           | All files      | pnpm commands and troubleshooting                  |
 | **git-workflow**         | All files      | Commit conventions, branch strategy, PR guidelines |
 | **development-workflow** | All files      | Code style, file naming, project conventions       |
+| **nix-workflow**         | All files      | Nix development environment and CI setup           |
 | **typescript-patterns**  | `**/*.ts`      | Type safety, exhaustiveness checks, clean code     |
 | **typescript-testing**   | `**/*.test.ts` | Vitest, MSW mocking, fs-fixture                    |
 | **file-operations**      | `**/*.ts`      | Native fetch API patterns and error handling       |