ci: revert checkout@v6 to v4

checkout@v6 is breaking CI on the Python sibling repo with exit code 1.
Reverting proactively here before it bites us on the next main push.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cubehouse

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       matrix:
         node: [18, 20, 22]
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
           node-version: ${{ matrix.node }}

.github/workflows/docs.yml

@@ -17,7 +17,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with: { node-version: '20', cache: 'npm' }
       - run: npm ci

.github/workflows/release.yml

@@ -13,7 +13,7 @@ jobs:
       contents: write
       id-token: write
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
           node-version: 20

.github/workflows/spec-drift.yml

@@ -14,7 +14,7 @@ jobs:
       pull-requests: write
       issues: write
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
           node-version: 20