fix: revert workflow checkout action to runloopai/checkout@main

Commit 80c9f2b accidentally replaced runloopai/checkout@main with
actions/checkout@v6 in ci.yml, publish-pypi.yml, and release-doctor.yml.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sid-rl

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ${{ github.repository == 'stainless-sdks/runloop-python' && 'depot-ubuntu-24.04' || 'ubuntu-slim' }}
     if: (github.event_name == 'push' || github.event.pull_request.head.repo.fork) && (github.event_name != 'push' || github.event.head_commit.message != 'codegen metadata')
     steps:
-      - uses: actions/checkout@v6
+      - uses: runloopai/checkout@main
 
       - name: Install uv
         uses: runloopai/setup-uv@main
@@ -46,7 +46,7 @@ jobs:
       id-token: write
     runs-on: ${{ github.repository == 'stainless-sdks/runloop-python' && 'depot-ubuntu-24.04' || 'ubuntu-slim' }}
     steps:
-      - uses: actions/checkout@v6
+      - uses: runloopai/checkout@main
 
       - name: Install uv
         uses: runloopai/setup-uv@main
@@ -84,7 +84,7 @@ jobs:
     runs-on: ${{ github.repository == 'stainless-sdks/runloop-python' && 'depot-ubuntu-24.04' || 'ubuntu-slim' }}
     if: github.event_name == 'push' || github.event.pull_request.head.repo.fork
     steps:
-      - uses: actions/checkout@v6
+      - uses: runloopai/checkout@main
 
       - name: Install uv
         uses: runloopai/setup-uv@main

.github/workflows/publish-pypi.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-slim
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: runloopai/checkout@main
 
       - name: Install uv
         uses: runloopai/setup-uv@main

.github/workflows/release-doctor.yml

@@ -12,7 +12,7 @@ jobs:
     if: github.repository == 'runloopai/api-client-python' && (github.event_name == 'push' || github.event_name == 'workflow_dispatch' || startsWith(github.head_ref, 'release-please') || github.head_ref == 'next')
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: runloopai/checkout@main
 
       - name: Check release environment
         run: |