chore(deps): bump actions/checkout to v7 (#91)

Applies dependabot #84 directly (it conflicted with the setup-node bump
that merged first). Bumps actions/checkout to v7 across all workflows.

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

Author: scalvert

.github/workflows/agent-baseline.yml

@@ -9,7 +9,7 @@ jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
       - uses: actions/setup-node@v6
         with:
           node-version: '20'

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: Setup mise
         uses: jdx/mise-action@v4
@@ -49,7 +49,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: Setup mise
         uses: jdx/mise-action@v4
@@ -71,7 +71,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: Setup mise
         uses: jdx/mise-action@v4