ci: bump workflow actions to Node 24-compatible majors

GitHub flagged the workflow actions as running on the deprecated Node 20
runtime (forced to Node 24 after 2026-06-16, removed 2026-09-16). Bump each
to its latest stable major:

- actions/checkout        v4 -> v6
- pnpm/action-setup       v3 -> v6  (version: 9 still passed explicitly)
- actions/setup-node      v4 -> v6  (node-version: 20 runtime unchanged)
- softprops/action-gh-release v2 -> v3

Our usage relies only on stable inputs, so no other changes are needed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ynamite

.github/workflows/ci.yml

@@ -11,13 +11,13 @@ jobs:
     timeout-minutes: 10
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
-      - uses: pnpm/action-setup@v3
+      - uses: pnpm/action-setup@v6
         with:
           version: 9
 
-      - uses: actions/setup-node@v4
+      - uses: actions/setup-node@v6
         with:
           node-version: 20
           cache: pnpm

.github/workflows/release.yml

@@ -20,13 +20,13 @@ jobs:
     timeout-minutes: 15
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
-      - uses: pnpm/action-setup@v3
+      - uses: pnpm/action-setup@v6
         with:
           version: 9
 
-      - uses: actions/setup-node@v4
+      - uses: actions/setup-node@v6
         with:
           node-version: 20
           cache: pnpm
@@ -115,7 +115,7 @@ jobs:
 
       - name: Create GitHub Release
         if: github.event_name == 'push'
-        uses: softprops/action-gh-release@v2
+        uses: softprops/action-gh-release@v3
         with:
           name: ${{ github.ref_name }}
           body: ${{ steps.notes.outputs.body }}