fix(action): detect existing Node ≥18 before calling setup-node

Previously the action's default node-version: '20' caused
actions/setup-node@v4 to silently downgrade callers who had pinned
Node 22+ via an earlier setup-node step.

Add a Detect existing Node.js step that checks `node` on PATH and its
major version. When Node ≥18 is already present we skip the inner
setup-node entirely; otherwise we still install the configured
node-version (default 20) as a fallback. Reword the input description
and update the README to document the new behavior.

Round-2 DevEx friction P1 — see /tmp/devex-r2-setup-devhelm/REPORT.md.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: caballeto

README.md

@@ -77,7 +77,7 @@ jobs:
 | `org-id` | No | — | Organization ID for multi-org accounts |
 | `workspace-id` | No | — | Workspace ID for multi-workspace accounts |
 | `verify-connection` | No | `false` | Run `devhelm auth me` after setup to verify credentials |
-| `node-version` | No | `20` | Node.js version. Set to `''` to skip if you manage Node yourself |
+| `node-version` | No | `20` | Node.js version to install **only if** none is detected on PATH (or if detected version is `<18`). Existing Node `>=18` is preserved as-is. Set to `''` to skip auto-install entirely |
 
 ## Outputs
 
@@ -88,12 +88,16 @@ jobs:
 
 ## How it works
 
-1. **Node.js** — ensures Node >= 18 is available (auto-installs via `actions/setup-node` unless you set `node-version: ''`)
+1. **Node.js** — detects an existing `node` on `PATH`; if it's `>=18`, the action keeps it and skips `actions/setup-node` entirely. Otherwise it installs `node-version` (default `20`) via `actions/setup-node`.
 2. **Cache** — restores `~/.npm` cache keyed on OS + CLI version
 3. **Install** — runs `npm install -g devhelm@<version>`
 4. **Environment** — exports `DEVHELM_API_TOKEN`, `DEVHELM_API_URL`, `DEVHELM_ORG_ID`, `DEVHELM_WORKSPACE_ID` for all subsequent steps
 5. **Verify** — optionally runs `devhelm auth me` to fail fast on bad credentials
 
+### Compatibility with caller-installed Node
+
+If your workflow already runs `actions/setup-node@v4` (or otherwise pins a Node version) before this action, **your Node version is preserved**. The action only installs Node when none is on `PATH` or the detected major is `<18`. You no longer need to set `node-version: ''` to avoid being silently downgraded.
+
 ## Security
 
 - **Never hardcode tokens** in workflow files. Use [GitHub Secrets](https://docs.github.com/en/actions/security-for-github-actions/security-guides/using-secrets-in-github-actions).
@@ -102,7 +106,7 @@ jobs:
 
 ## Requirements
 
-- **Node.js >= 18** — the action auto-installs Node 20 by default. Set `node-version: ''` if your workflow already provides Node.
+- **Node.js >= 18** — if not present (or the detected version is `<18`), the action installs Node 20 by default. Any pre-existing Node `>=18` set up by an earlier step is left untouched. Set `node-version: ''` to skip the install fallback entirely (the action will then error if Node is missing).
 - **npm** — ships with Node.js.
 
 ## License

action.yml

@@ -28,7 +28,7 @@ inputs:
     required: false
     default: 'false'
   node-version:
-    description: 'Node.js version to use if actions/setup-node was not called before this action. Set to empty string to skip auto-setup.'
+    description: 'Node.js version to install IF none is detected on PATH (or if detected version is <18). Existing Node >=18 is preserved. Set to empty string to skip auto-setup entirely. Default: 20.'
     required: false
     default: '20'
 
@@ -43,21 +43,39 @@ outputs:
 runs:
   using: composite
   steps:
+    - name: Detect existing Node.js
+      id: node-detect
+      shell: bash
+      run: |
+        if command -v node >/dev/null 2>&1; then
+          MAJOR=$(node -v | sed 's/^v\([0-9]*\).*/\1/')
+          if [ "$MAJOR" -ge 18 ]; then
+            echo "found=true" >> "$GITHUB_OUTPUT"
+            echo "version=$(node -v)" >> "$GITHUB_OUTPUT"
+            echo "Node $(node -v) detected (>=18); skipping setup-node."
+            exit 0
+          fi
+          echo "Node $(node -v) detected but <18; will install Node ${{ inputs.node-version }}."
+        else
+          echo "No Node.js detected on PATH; will install Node ${{ inputs.node-version }}."
+        fi
+        echo "found=false" >> "$GITHUB_OUTPUT"
+
     - name: Ensure Node.js is available
-      if: inputs.node-version != ''
+      if: steps.node-detect.outputs.found != 'true' && inputs.node-version != ''
       uses: actions/setup-node@v4
       with:
         node-version: ${{ inputs.node-version }}
 
     - name: Validate Node.js
       shell: bash
       run: |
-        if ! command -v node &>/dev/null; then
-          echo "::error::Node.js is required. Either set input 'node-version' or run actions/setup-node before this action."
+        if ! command -v node >/dev/null 2>&1; then
+          echo "::error::Node.js is required. Either set input 'node-version' (default: 20) or run actions/setup-node before this action."
           exit 1
         fi
-        NODE_MAJOR=$(node -v | sed 's/v\([0-9]*\).*/\1/')
-        if (( NODE_MAJOR < 18 )); then
+        NODE_MAJOR=$(node -v | sed 's/^v\([0-9]*\).*/\1/')
+        if [ "$NODE_MAJOR" -lt 18 ]; then
           echo "::error::DevHelm CLI requires Node.js >= 18 (found $(node -v))"
           exit 1
         fi