ci(actions): Update actions/checkout v5 to v6

why: Security and feature updates
what:
- Update actions/checkout from v5 to v6

refs:
- https://github.com/actions/checkout/releases/tag/v6.0.0

Author: tony

.github/workflows/docs.yml

@@ -17,7 +17,7 @@ jobs:
       matrix:
         python-version: ['3.14']
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Filter changed file paths to outputs
         uses: dorny/paths-filter@v3.0.2

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
         python-version: ['3.14']
         tmux-version: ['3.2a', '3.3a', '3.4', '3.5', '3.6', 'master']
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Install uv
         uses: astral-sh/setup-uv@v7
@@ -100,7 +100,7 @@ jobs:
         python-version: ['3.14']
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Install uv
         uses: astral-sh/setup-uv@v7