ci: drop cache:npm from setup-node so it doesn't hit EBADENGINE on runner

The forgejo-runner pod bootstraps with nodejs-slim_22 from nix (so JS-based
Forgejo Actions can launch). setup-node@v4 with `cache: npm` invokes system
npm — under Node 22 — which fails the engines check ("Required: >=26.1.0,
Actual: v22.22.3") before any workflow step ever runs.

The downstream `npm ci` step runs after setup-node updates PATH to the
just-installed Node 26.1.0, so it works fine. We're just losing the
auto-set-up npm download cache here; can wire SF's own cache later if first
runs feel slow.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.forgejo/workflows/self-deploy.yml

@@ -36,7 +36,13 @@ jobs:
         uses: actions/setup-node@v4
         with:
           node-version: ${{ env.NODE_VERSION }}
-          cache: npm
+          # cache: npm intentionally omitted — setup-node@v4 invokes the system
+          # npm to validate the lockfile, and on the forgejo-runner pod the
+          # system Node is v22 (the slim nix-profile install used to bootstrap
+          # JS-based actions). package.json's engines: { node: ">=26.1.0" }
+          # triggers EBADENGINE on that v22 invocation. The workflow's own
+          # `npm ci` step below runs after PATH is updated to the just-installed
+          # Node 26, so it's unaffected.
 
       - name: Install dependencies
         run: |