fix(ci): remove SIGKILL fallback from check-dagger cleanup

The GET /shutdown endpoint on otel-receiver.py is the one clean shutdown
path. cleanup() only needs to remove temp files.

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

Taskfile.yml

@@ -294,7 +294,6 @@ tasks:
         python3 ci/otel-receiver.py --port-file="$PORTFILE" &
         RECV_PID=$!
         cleanup() {
-          kill -9 "$RECV_PID" 2>/dev/null || true
           rm -f "$PORTFILE" "$DAGGER_OUT" "$RC_FILE"
         }
         trap cleanup EXIT