chore: externalize configure-nodejs dogfood coverage

.gitignore

@@ -1,7 +1 @@
-node_modules/
 .DS_Store
-
-fixtures/*/node_modules/
-fixtures/*/.pnp.*
-fixtures/*/.yarn/install-state.gz
-fixtures/*/.yarn/cache/

README.md

@@ -52,7 +52,7 @@ steps:
 | Input | Default | Description |
 | --- | --- | --- |
 | `node-version` | `22.x` | Node.js version to install with `actions/setup-node` |
-| `package-manager` | `""` | Explicit override for `npm`, `pnpm`, or `yarn` |
+| `package-manager` | `""` | Optional override for `npm`, `pnpm`, or `yarn`; by default the action follows the package manager inferred from `package.json` and the lockfile present |
 | `working-directory` | `"."` | Repository-relative directory containing `package.json` and the lockfile |
 | `cache-key-suffix` | `""` | Optional suffix appended to the dependency cache key when you want to namespace cache entries |
 | `lookup-only` | `"false"` | When `true`, only checks whether the cache exists and skips downloading it |
@@ -100,19 +100,15 @@ The first release targets Yarn repositories that install into `node_modules`.
 
 - Yarn 2+ uses `yarn install --immutable`
 - Yarn 1 uses `yarn install --frozen-lockfile`
-- the fixture coverage in this repository uses Yarn 4 with `nodeLinker: node-modules`
+- the dogfood coverage in `pwrdrvr/configure-nodejs-test` uses Yarn 4 with `nodeLinker: node-modules`
 
 Plug'n'Play-specific caching is intentionally out of scope for `v1`.
 
 ## Development
 
-This repository includes three end-to-end fixtures under `fixtures/`:
+Dogfood coverage for this action lives in [`pwrdrvr/configure-nodejs-test`](https://github.com/pwrdrvr/configure-nodejs-test).
 
-- `fixtures/npm-basic`
-- `fixtures/pnpm-basic`
-- `fixtures/yarn-basic`
-
-The CI workflow runs local unit tests plus end-to-end matrix coverage for those fixtures, followed by cache-lookup verification in a second matrix job.
+That repository checks out `pwrdrvr/configure-nodejs` at a configurable ref, runs the unit tests for the helper scripts, and exercises npm, pnpm, and Yarn fixtures through the action entrypoint.
 
 ## Releases
 

action.yml

@@ -6,7 +6,7 @@ inputs:
     description: "Node.js version to install"
     default: "22.x"
   package-manager:
-    description: "Explicit package manager override (npm, yarn, pnpm)"
+    description: "Optional override for npm, yarn, or pnpm; defaults to the package manager inferred from package.json and lockfiles"
     default: ""
   working-directory:
     description: "Repository-relative directory containing package.json and lockfile"
@@ -84,7 +84,7 @@ runs:
 
     - name: Restore dependencies from cache
       id: cache-node-modules
-      uses: actions/cache@v5
+      uses: actions/cache/restore@v5
       with:
         path: ${{ steps.resolve-cache-paths.outputs.cachePaths }}
         key: node-modules-${{ inputs.node-version }}-${{ runner.os }}-${{ runner.arch }}-${{ steps.resolve-cache-paths.outputs.workingDirectoryKey }}-${{ steps.resolve-package-manager.outputs.managerCacheKey }}-${{ steps.resolve-package-manager.outputs.lockfileSha }}${{ steps.resolve-cache-paths.outputs.cacheKeySuffixSegment }}
@@ -95,3 +95,22 @@ runs:
       shell: bash
       working-directory: ${{ steps.resolve-cache-paths.outputs.workingDirectory }}
       run: ${{ steps.resolve-package-manager.outputs.installCommand }}
+
+    - name: Detect cacheable dependency paths
+      id: detect-cache-paths
+      if: steps.cache-node-modules.outputs.cache-hit != 'true'
+      shell: bash
+      working-directory: ${{ steps.resolve-cache-paths.outputs.workingDirectory }}
+      run: |
+        if find . -type d -name node_modules -print -quit | grep -q .; then
+          echo "exists=true" >> "$GITHUB_OUTPUT"
+        else
+          echo "exists=false" >> "$GITHUB_OUTPUT"
+        fi
+
+    - name: Save dependencies to cache
+      if: steps.cache-node-modules.outputs.cache-hit != 'true' && steps.detect-cache-paths.outputs.exists == 'true'
+      uses: actions/cache/save@v5
+      with:
+        path: ${{ steps.resolve-cache-paths.outputs.cachePaths }}
+        key: node-modules-${{ inputs.node-version }}-${{ runner.os }}-${{ runner.arch }}-${{ steps.resolve-cache-paths.outputs.workingDirectoryKey }}-${{ steps.resolve-package-manager.outputs.managerCacheKey }}-${{ steps.resolve-package-manager.outputs.lockfileSha }}${{ steps.resolve-cache-paths.outputs.cacheKeySuffixSegment }}