ci: bump actions to Node 24 runtimes and drop EOL Node 20

actions/checkout@v4 and actions/setup-node@v4 target the deprecated Node 20
runner runtime, so every run logs forced-Node-24 deprecation annotations.
Harmonize checkout on the v6 major already used by the other three jobs,
bump setup-node to v6, and move node-version from Node 20 (EOL 2026-04-30)
to Node 24 LTS.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/test-plugin-install.yml

@@ -13,9 +13,9 @@ jobs:
       - uses: actions/checkout@v6
 
       - name: Set up Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
-          node-version: '20'
+          node-version: '24'
 
       - name: Validate all skills
         run: node scripts/validate-skills.js
@@ -30,12 +30,12 @@ jobs:
     name: Validate command parity and description sync
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
-          node-version: '20'
+          node-version: '24'
 
       - name: Validate commands across all tool directories
         run: node scripts/validate-commands.js