Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/gemini-invoke.yml

@@ -38,7 +38,7 @@ jobs:
           permission-pull-requests: 'write'
 
       - name: 'Checkout Code'
-        uses: 'actions/checkout@v4' # ratchet:exclude
+        uses: 'actions/checkout@v6'
 
       - name: 'Run Gemini CLI'
         id: 'run_gemini'

.github/workflows/gemini-plan-execute.yml

@@ -40,7 +40,7 @@ jobs:
           permission-pull-requests: 'write'
 
       - name: 'Checkout Code'
-        uses: 'actions/checkout@v4' # ratchet:exclude
+        uses: 'actions/checkout@v6'
 
       - name: 'Run Gemini CLI'
         id: 'run_gemini'