fix(container): use detached checkout for shallow clones

gh pr checkout fails with --depth=1 clones on gh CLI 2.90.0:
"fatal: cannot set up tracking information; starting point is not a branch"

The --detach flag avoids setting up a tracking branch, which is
unnecessary since the container only needs the PR files on disk.

Also fixes pre-existing ruff format issue in test_sessions_router.py.

Author: mariuspruvot

apps/api/tests/modules/installation/test_sessions_router.py

@@ -249,9 +249,7 @@ async def test_session_response_fields(self, authed_client_with_installation):
         client, installation = authed_client_with_installation
 
         with _mock_github_admin():
-            response = await client.get(
-                f"/api/v1/installations/{installation.github_installation_id}/sessions"
-            )
+            response = await client.get(f"/api/v1/installations/{installation.github_installation_id}/sessions")
 
         assert response.status_code == 200
         item = response.json()["items"][0]

infra/docker/claude-runner/entrypoint.sh

@@ -59,7 +59,7 @@ wait $diff_pid || {
 
 # Checkout PR branch (requires completed clone)
 cd /workspace
-gh pr checkout "$PR_NUMBER" || {
+gh pr checkout "$PR_NUMBER" --detach || {
   emit_error "Failed to checkout PR #${PR_NUMBER}. The branch may have been deleted after the PR was merged."
   exit 1
 }