fix(ci): drop actions/setup-node — use runner image's node (fixes ETXTBSY)

actions/setup-node writes node into a tool-cache shared across concurrent jobs;
with capacity>1 one job execs node while another writes it → "/usr/bin/env:
'node': Text file busy". The catthehacker runner image already ships node 24,
and corepack (bundled) reads each app's packageManager — so setup-node is
unneeded. Removing it eliminates the shared-cache race.

.gitea/workflows/ci.yml

@@ -25,13 +25,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: actions/setup-node@v4
-        with:
-          node-version: 22
-
-      # Use corepack (bundled with node) instead of pnpm/action-setup: it reads
-      # each app's packageManager from its own package.json, which is what this
-      # per-app monorepo (no root package.json) needs.
+      # Node comes from the runner image (catthehacker ships node 24) — NOT
+      # actions/setup-node, whose shared tool-cache races across concurrent jobs
+      # ("node: Text file busy"). corepack (bundled with node) reads each app's
+      # own packageManager — what this per-app monorepo (no root package.json) needs.
       - name: Install
         run: |
           corepack enable
@@ -47,9 +44,6 @@ jobs:
         working-directory: services/platform-api
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-node@v4
-        with:
-          node-version: 22
       - name: Install
         run: |
           corepack enable