ci: upgrade actions/checkout and actions/setup-node to v6 (Node 24)

Replace v4 (Node 20) with v6 (Node 24 native) to eliminate the
deprecation warning. Remove the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24
workaround since v6 doesn't need it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -5,19 +5,16 @@ on:
     branches: [master, main]
   pull_request:
 
-env:
-  FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
-
 jobs:
   ci:
     name: Typecheck & Lint
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 22
           cache: 'npm'

.github/workflows/release.yml

@@ -7,9 +7,6 @@ on:
 
 concurrency: ${{ github.workflow }}-${{ github.ref }}
 
-env:
-  FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
-
 jobs:
   release:
     name: Release
@@ -19,10 +16,10 @@ jobs:
       pull-requests: write
     steps:
       - name: Checkout repo
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Setup Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 22
           cache: npm